New feature: eigenfunction subset

[n-claes]

PR description

This PR adds the possibility to select which eigenfunctions need to be saved to the datfile. Previously all eigenfunctions were automatically saved, including the ones from modes that were far from resolved. This is a needless waste of datfile storage since those eigenfunctions will never be used.

Here we add some flags to the savelist namelist, where one can now provide a point in the complex plane and draw a circle around it, and only the eigenvalues inside that circle will have their eigenfunctions saved.

This PR does a complete overhaul of how eigenfunctions are treated (both for the base eigenfunctions as derived quantities thereof). Base eigenfunctions and derived quantities are now split up into submodules, eigenfunction assembly has been simplified.

Note: this PR introduces use of the Fortran intrinsic findloc, which is available from gfortran v9.0 onwards. So unless we drop support for lower gfortran versions, we should implement a workaround findloc function by simply looping through the arrays.

TODO code

• code cleaning: some things (especially the derived eigenfunction submodule) need to be cleaned, similar routines need to be merged together in appropriate procedures for easier maintenance.
• refactoring: some routine names/function names need to be changed: "postprocessed" -> "derived_eigenfunctions"? confusion with directory "post_processing" already present... Also, these are derived quantities and calculated on-the-fly along with the regular eigenfunctions and not during postprocessing.

TODO tests

• add regression test(s) for the derived quantities
• add unit tests
• add pylbo tests
• make sure everything new that has been added is also covered

[codecov]

Codecov Report

Merging #75 (f862e0b) into develop (d10a3a3) will increase coverage by 5.41%.
The diff coverage is 85.99%.

@@             Coverage Diff             @@
##           develop      #75      +/-   ##
===========================================
+ Coverage    81.46%   86.88%   +5.41%     
===========================================
  Files           99      102       +3     
  Lines         5876     5885       +9     
===========================================
+ Hits          4787     5113     +326     
+ Misses        1089      772     -317     

Flag	Coverage Δ
legolas	88.74% <99.10%> (+7.42%)	⬆️
pylbo	83.14% <61.45%> (+1.37%)	⬆️

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