Add a few python2 packages for scientific computing

See merge request !1

* modules/past/packages/chemistry.scm (nmoldyn): New variable.

Package references via specifications are not accessible to input
rewriting. For the inputs that are defined in guix-past rather than
guix, direct references have no disadvantage and add flexibility.
In particular, this patch permits building matplotlib with
numpy-1.8.

Contains package "domainfinder"

* modules/past/packages/chemistry.scm: New file.

* modules/past/packages/python27.scm (python2-mmtk): New variable.

* modules/past/packages/python27.scm (python2-scientific): New variable.

* modules/past/packages/python27.scm (python2-pyro): New variable.

* modules/past/packages/python27.scm (python2-numpy-1.8): New variable.

* modules/past/packages/python27.scm (python2-scipy): Update to 1.2.3.
[source]: Add snippet to remove cythonized files.
[arguments]: Add phase to cythonize files.

* modules/past/packages/python27.scm (python2-numpy): Update to 1.16.6.
[source]: Add snippet to remove cythonized file.
[arguments]: Add phase to generate cythonized file.

* modules/past/packages/python27.scm (python2-pandas): New variable.

* modules/past/packages/python27.scm (python2-beautifulsoup4): New
variable.

* modules/past/packages/python27.scm (python2-lxml): New variable.

* modules/past/packages/python27.scm (python2-openpyxl): New variable.

* modules/past/packages/python27.scm (python2-soupsieve): New variable.

* modules/past/packages/python27.scm (python2-pytest-mock): New variable.

* modules/past/packages/python27.scm (python2-mock): New variable.

* modules/past/packages/python27.scm (python2-singledispatch): New
variable.

* modules/past/packages/bioinformatics.scm (bamm): New variable.

* modules/past/packages/python27.scm (python2-package): Export it.

Later versions cannot be built with Python 2.

* modules/past/packages/python27.scm (python2-setuptools): Revert to
41.0.1.
[native-inputs]: Add unzip.

* modules/past/packages/python27.scm (python2-setuptools)[arguments]:
Add #:python argument.

* modules/past/packages/python27.scm (python2-nose): New variable.

* modules/past/packages/python27.scm (python2-cython)[arguments]: Set
 #:python argument.

* modules/past/packages/python27.scm (python2-setuptools): New variable.

* modules/past/packages/python27.scm (python2-scipy): New variable.

* modules/past/packages/python27.scm (python2-matplotlib): New variable.

* modules/past/packages/python27.scm (python2-dateutil): New variable.

* modules/past/packages/python27.scm (python2-pycairo): New variable.

* modules/past/packages/python27.scm (python2-subprocess32): New
variable.
* past/patches/python2-subprocess32-disable-input-test.patch: New file.

* modules/past/packages/python27.scm (python2-pygobject-2): New
variable.
* past/patches/python2-pygobject-2-deprecation.patch: New file.

* modules/past/packages/python27.scm (python2-functools32): New variable.

* modules/past/packages/python27.scm (python2-backports-functools-lru-cache):
New variable.

* modules/past/packages/python27.scm (python2-hypothesis): New variable.

* modules/past/packages/python27.scm (python2-pytest-cov): New variable.

* modules/past/packages/python27.scm (python2-enum34): New variable.

* modules/past/packages/python27.scm (python2-pillow)[arguments]: Use
python-2.

* modules/past/packages/python27.scm (python2-pillow): New variable.

* modules/past/packages/bioinformatics.scm: Use (past packages
python27).

Notably, this adds python2-numpy.

* modules/past/packages/python27.scm: New file.