Make template interface the default

[EmilyBourne]

In order to remove inheritance the GMGPolar class is templated on a certain number of parameters. This is fine for simulations (e.g. gyselalibxx) that want to use the library, but it causes issues for the command line interface. Up to now we used a pointer to a superclass to avoid this problem. However in order to template the Level on the geometry, the function solve will need to be in the subclass. It is not possible to write an abstract interface for a templated function so this solution will no longer be possible. This PR uses std::visit to create an instance of the GMGPolar class. This slows down the compilation as an instance of the class must be created for each configuration that the ConfigParser describes.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.91%. Comparing base (18d7d76) to head (9df2877).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #189   +/-   ##
=======================================
  Coverage   94.91%   94.91%           
=======================================
  Files          94       94           
  Lines        9516     9516           
=======================================
  Hits         9032     9032           
  Misses        484      484           

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