Move choice of floating-point precision to the Python layer

[JonathanMaes]

This PR simplifies the process of switching between single and double floating-point precision.
Previously, the precision had to be chosen before compilation, by editing CMakeLists.txt.
Now, mumax+ compiles two binaries: one for single and one for double precision (unless MUMAXPLUS_FP_PRECISION is set, see below). The appropriate binary is then loaded during the import mumaxplus statement.

Two switches control the floating-point precision, as listed below in descending order of priority. Both can take the values SINGLE/1/32 for single precision and DOUBLE/2/64 for double precision.

• A command-line flag --mumaxplus-fp-precision passed to the python command.
  • Example: python some_mumax_script.py --mumaxplus-fp-precision DOUBLE.
• The environment variable MUMAXPLUS_FP_PRECISION.
  • Setting this will also affect the compilation: mumax+ will then only be compiled for one kind of floating point precision. This can be useful to reduce compile time during development, but passing the opposite precision with --mumaxplus-fp-precision will then result in a ModuleNotFoundError.

The used precision can then be accessed as mumaxplus.FP_PRECISION.

Note

Since the choice of precision must be made before the import mumaxplus statement, it is not possible to use single precision in one part of a script and double precision in another: these must be run in separate processes. Reloading mumax+ with a different floating point precision during runtime may lead to a vast wealth of errors.

The unit test is based on examples/errorscaling.py, since the error scaling is highly dependent on the floating point precision.

[ilateur]

Someone (or some-multiple) should test this on linux

[DiegoDeGusem]

On my linux device the code compiles and the test succeeds 🎉
I also agree with @ilateur's comment about a larger description.

[JonathanMaes]

A webpage was added in the Tutorials section, explaining how to choose a precision, what errors can occur, and showcasing the effect on the error floor.