Fix typos in how to run check_2d

Add a function (ptr_on_variables()) to access the tuple pointing to the first element of the variables in the particle block

 Use tree.hpp

* fix/clean-up: (81 commits)
  added a call to memory_usage
  Setting the wrong max_level following a debugging session
  Added function to compute the memory usage of the interpolator
  Added kernel 2 et 3
  added a python interface for reading and writing files
  guix: add minimal guix set-up to build the documentation using guix
  guix: Update guix manifest files to include 'openmpi'
  cmake: Update 'CMakePresets.json' file to take MPI into account
  improvement in fmawriter for Python
  add doc
  add ascii writer
  mpi compiles and the tests are Ok
  The no mpi part compile and the tests are ok
  add sort include
  The proc_task algorithm seems to work
  Fix 2d problem with max_level when sorting particles (max_level is now bounded by 30)
  Adding dependencies to serialize communications in the L2L operator
  add debug outputs
  Add outputs
  Check missing indexes
  ...

Merge branch 'fix/clean-up' of https://gitlab.inria.fr/solverstack/ScalFMM into feature/3.0/python-binding

Update ci environment and jobs

See merge request solverstack/ScalFMM!25

build directory is too large (2Go) for gitlab's artifacts with these flags, remove them and comment code coverage reporting (flags are necessary to it)