Transpilation: optional arguments

[MichaelSt98]

Allow/handle optional arguments for c-like-backends.
For

• cgen - assume all optional arguments are passed (inherently unsafe, thus warn)
• cppgen (and all backends derived from that) - proper handling via optional argument with default value nullptr
  • arguments that would have been passed by value are instead passed as pointer if they are optional

Moreover, introduced capabilities to jit compile C++ files (in addition to C files).

[github-actions]

Documentation for this branch can be viewed at https://sites.ecmwf.int/docs/loki/385/index.html

[codecov]

Codecov Report

Attention: Patch coverage is 99.13043% with 1 line in your changes missing coverage. Please review.

Project coverage is 95.49%. Comparing base (cc179b3) to head (3e6e7b8).
Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
loki/build/compiler.py	94.11%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #385      +/-   ##
==========================================
+ Coverage   95.48%   95.49%   +0.01%     
==========================================
  Files         185      185              
  Lines       38646    38744      +98     
==========================================
+ Hits        36902    37000      +98     
  Misses       1744     1744              

Flag	Coverage Δ
lint_rules	96.39% <ø> (ø)
loki	95.48% <99.13%> (+0.01%)	⬆️

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[reuterbal]

Very neat! A few minor nitpicks but otherwise good to go

[reuterbal]

Many thanks, looks great! I'll do a rebase and merge once green

[mlange05]

Very nice! Looks great, GTG.