(Closes #2202) Update the generation of Fortran declarations to observe dependencies

[arporter]

This is a relatively simple PR - it takes the inter-dependence handling out of _gen_param_decls and puts it into gen_decls. This also then means we can delete _gen_param_decls. I've added tests for the various tricky cases described in the original Issue and they all pass now :-)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.95%. Comparing base (214ae5a) to head (72f247e).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3344      +/-   ##
==========================================
- Coverage   99.95%   99.95%   -0.01%     
==========================================
  Files         384      384              
  Lines       54214    54205       -9     
==========================================
- Hits        54192    54183       -9     
  Misses         22       22              

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[arporter]

The NEMO ITs failed because of character variables:

Symbol Table of Container 'test_mod':
-------------------------------------
DataSymbol:
  cdwmo: DataSymbol<UnsupportedFortranType('CHARACTER(LEN = ilenwmo), DIMENSION(:), POINTER, PUBLIC :: cdwmo'), Unknown>
  ilenwmo: DataSymbol<Scalar<INTEGER, UNDEFINED>, Static, initial_value=Literal[value:'58', Scalar<INTEGER, UNDEFINED>], constant=True>

(Pdb) cdwmo = table.lookup("cdwmo")
(Pdb) cdwmo.get_all_accessed_symbols()
set()
(Pdb) cdwmo.datatype.declaration
'CHARACTER(LEN = ilenwmo), DIMENSION(:), POINTER, PUBLIC :: cdwmo'

I was proceeding to try and do something with the parse tree in UnsupportedFortranType but actually, it would probably be better to work on populating the partial_datatype in this case.

[arporter]

Marking this as blocked for the moment because I want to investigate adding support for a CharacterType with a length property.