Refactoring of CField struct to include gridindexingtype and interp_method

[erikvansebille]

Following #1612, where we need to provide the gridindexingtype on the CField struct for technical reasons, it makes sense to also use it throughout the rest of the C-code. Hence, this PR refactors the code to not require the gridindexingtype and interp_method when calling a Field evaluation in C, but get them from the CField itself

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Additional details and impacted files

@@                     Coverage Diff                     @@
##           analyticaladvection_in_jit    #1673   +/-   ##
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  Coverage                       86.22%   86.22%           
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  Files                              67       67           
  Lines                           11827    11828    +1     
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+ Hits                            10198    10199    +1     
  Misses                           1629     1629           

Flag	Coverage Δ
integration-tests	64.85% <100.00%> (+<0.01%)	⬆️
unit-tests	80.62% <100.00%> (+<0.01%)	⬆️

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Files with missing lines	Coverage Δ
parcels/field.py	89.72% <100.00%> (+<0.01%)	⬆️

[erikvansebille]

This is still a bit ugly, because it mirrors the information from

Parcels/parcels/include/index_search.h

Lines 22 to 30 in 997e3f8

	typedef enum
	{
	LINEAR=0, NEAREST=1, CGRID_VELOCITY=2, CGRID_TRACER=3, BGRID_VELOCITY=4, BGRID_W_VELOCITY=5, BGRID_TRACER=6, LINEAR_INVDIST_LAND_TRACER=7, PARTIALSLIP=8, FREESLIP=9
	} InterpCode;
	typedef enum
	{
	NEMO = 0, MITGCM = 1, MOM5 = 2, POP = 3
	} GridIndexingType;

It would be cleaner to either

1. Use the string here in the CField, instead of an integer. I tried using c_char_p, but that didn't work (yet)
2. Somehow import the typedefs from index_search.h (above) here
   In any case, I think we want to avoid happing two places where these mappings between strings and integers occur...