Use operator() instead of rhs_f function in SourceTerms

[EmilyBourne]

Use operator()(std::size_t i_r, std::size_t i_theta) instead of rhs_f(double r, double theta) function in SourceTerms. In Pyccel the RHS is known at the grid points. If we pass integers then this information can be used directly. Otherwise an unnecessary interpolation must be added to make the RHS available at every point in space. Otherwise a function must be added to find the nearest grid point to the coordinate. This is also costly when handling non-uniform points. The function is renamed to operator() as this means that when inheritance is removed (see #131) a Kokkos::Vector would be accepted as a SourceTerm. This removes unnecessary function calls.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.29%. Comparing base (82c83f2) to head (69bc471).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #138   +/-   ##
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  Coverage   88.28%   88.29%           
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  Files          89       89           
  Lines        5276     5280    +4     
=======================================
+ Hits         4658     4662    +4     
  Misses        618      618           

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[julianlitz]

There is only a single SourceTermFunctions instance available. Since it is used exclusively at level_depth = 0 we can provide the grid at that level and your approach works perfectly as implemented here.

However, this assumption may not hold for DomainGeometry or DensityProfiles if they are invoked across multiple refinement levels (when we would switch to integers there as well).

In those cases, one option would be to always provide the grid at level = 0 and provide an additional argument
operator(i_r, i_theta, level_depth)
and convert internally
double r = grid.radius(i_r << level_depth);
double theta = grid.radius(i_theta << level_depth);.

Another option would be to make PolarGrid a template parameter of
operator<PolarGrid>(i_r, i_theta),
though this could potentially conflict with Kokkos.

Another option:
operator(const PolarGrid& grid, int i_r, int i_theta);

1 Seems a bit hacky
2 Best performance but strange looking template
3 Clear and easy but additional argument