Compile with language C by default

The default language is now "c". Solves #214 (closed)

This should enable easier use of Struphy with macOS.

OpenMP for both PIC and FEEC is disabled by default and has to be called with the respective flags. The "--compiler" option can now take the four compilers available in pyccel ("GNU" is the default, "intel", "PGI", "nvidia"). Moreover, a JSON file can be specified to define a custom compiler (see pyccel doc).

It was necessary to add temporaries for c-slicing to avoid memory leaks in some kernels (in particular for geometry). Moreover, we now enforce c-ordering in _evaluate_metric_coefficient.

Other changes:

• Removed warnings due to sqrt() in pusher_utilities.py compilation
• Remove most restrictions in pyproject.toml, require pyccel>=1.10.0. Solves #216 (closed)
• Added some int() conversions in order to be able to run with only part of files compiled.
• the psydac version has been upgraded to 0.1.3 - it also compiles with c now.