Enhance NSIS installer by using py launcher instead of python.

No need to find the good version of python in PATH, it is enough to just install it (the py launcher knows where to find it).

misc/nsis/faust.nsi

@@ -125,18 +125,29 @@ Section "" ; no component so name not needed
   ; TODO: fix this issue directly in CMake (if possible)
   RMDir /r "$INSTDIR\matlab\expfiltered"
 
-  ; check the python version matches python major.minor version used to build the the wrapper shared library
-  Exec "python --version | python -c $\"import re; ver = input(); exit(0) if re.match('Python @WIN_PY_VER@', ver) else exit(1)$\""
+;  ; check the python version matches python major.minor version used to build the the wrapper shared library
+;  Exec "python --version | python -c $\"import re; ver = input(); exit(0) if re.match('Python @WIN_PY_VER@', ver) else exit(1)$\""
+;  IfErrors 0 +2
+;  MessageBox MB_OK "Error: this version of FAuST is pre-compiled for Python @WIN_PY_VER@ which must be installed and configured as the default python on your system (i.e. it must be available as $\"python$\" command in the PATH environment variable)." /SD IDOK IDOK data_dl
+;  MessageBox MB_OK "The pyfaust wrapper will be installed for Python @WIN_PY_VER@." /SD IDOK
+
+  Exec "py -@WIN_PY_VER@ -c $\"print()$\""
   IfErrors 0 +2
-  MessageBox MB_OK "Error: this version of FAuST is pre-compiled for Python @WIN_PY_VER@ which must be installed and configured as the default python on your system (i.e. it must be available as $\"python$\" command in the PATH environment variable)." /SD IDOK IDOK data_dl
+  MessageBox MB_OK "Error: this version of FAuST is pre-compiled for Python @WIN_PY_VER@ which must be installed and available through the py command wrapper which must have been installed too (https://docs.python.org/3/using/windows.html#launcher)." /SD IDOK IDOK data_dl
   MessageBox MB_OK "The pyfaust wrapper will be installed for Python @WIN_PY_VER@." /SD IDOK
 
   ; post install pyfaust auto-setup in environment (works only if python is installed in path)
   ${StrRep} '$0' $TEMP '\' '\\'
-  Exec "python -c $\"import site;dir=site.getsitepackages()[1];f=open('$0\\tmp_site_pkg', 'w');f.write(dir);f.close()$\""
+;  Exec "python -c $\"import site;dir=site.getsitepackages()[1];f=open('$0\\tmp_site_pkg', 'w');f.write(dir);f.close()$\""
+;  IfErrors 0 +2
+;  MessageBox MB_OK "Error: no python found into your PATH environment variable. You'll have to do the Faust setup manually (you'll see how in the documentation)." /SD IDOK IDOK data_dl
+;  MessageBox MB_OK "Faust installed in your python environment (the version found into your PATH environment variable)." /SD IDOK
+
+  Exec "py -@WIN_PY_VER@ -c $\"import site;dir=site.getsitepackages()[1];f=open('$0\\tmp_site_pkg', 'w');f.write(dir);f.close()$\""
   IfErrors 0 +2
-  MessageBox MB_OK "Error: no python found into your PATH environment variable. You'll have to do the Faust setup manually (you'll see how in the documentation)." /SD IDOK IDOK data_dl
-  MessageBox MB_OK "Faust installed in your python environment (the version found into your PATH environment variable)." /SD IDOK
+  MessageBox MB_OK "Error: py not found for this version or for any version." /SD IDOK IDOK data_dl ; can't happen since it was already tested above (as success or exit)
+  MessageBox MB_OK "Faust installed in your python environment (version @WIN_PY_VER@)." /SD IDOK
+
 
   FileOpen $1 "$TEMP\tmp_site_pkg" r
   FileRead $1 $2
@@ -166,7 +177,7 @@ Section "" ; no component so name not needed
   FileWrite $1 "$\r$\n_NSI_INSTALL_PATH='$INSTDIR'"
   FileClose $1
 
-  ExecWait "python -m pip install @PYFAUST_PYTHON_REQUIREMENTS@"
+  ExecWait "py -@WIN_PY_VER@ -m pip install @PYFAUST_PYTHON_REQUIREMENTS@"
   ; ExecWait doesn't work with this command, if eventually pip install command fails, the user will be noticed when importing pyfaust
   ;IfErrors 0 +2
   ;MessageBox MB_OK "Error: failed partly or totally to install the pyfaust python packages through pip, please install them manually to get a workable pyfaust, list of packages: @PYFAUST_PYTHON_REQUIREMENTS@." IDOK data_dl
@@ -178,10 +189,10 @@ Section "" ; no component so name not needed
   ; create data folder
   ; try with python first
   ${StrRep} '$3' $INSTDIR '\' '\\'
-  Exec "python -c $\"from os import mkdir; mkdir('$3\\matlab\\data')$\""
+  Exec "py -@WIN_PY_VER@ -c $\"from os import mkdir; mkdir('$3\\matlab\\data')$\""
   ; download data
   ClearErrors ; in case the data dir was already existing
-  Exec "python $2\pyfaust\datadl.py $\"$INSTDIR\matlab\data$\""
+  Exec "py -@WIN_PY_VER@ $2\pyfaust\datadl.py $\"$INSTDIR\matlab\data$\""
   IfErrors 0 after_data_dl
   MessageBox MB_OK "Downloading FAuST data with python seems to have failed (or maybe it's already done), now trying with powershell." /SD IDOK
   ClearErrors