Minor cleanup of Fortran binding stuff to simplify future maintainance

There is no distinction between F90 and non-F90 code so combine ftn-custom and f90-custom directories and fortranimpl.h and f90

Minor cleanup of Fortran binding stuff to simplify future maintainance

There is no distinction between F90 and non-F90 code so combine ftn-custom and f90-custom directories and fortranimpl.h and f90impl.h files. Also move uses of f90 to ftn when simple

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This requires some changes to user FORTRAN code

Pass PETSC_NULL_XXX_ARRAY when passing a NULL as an argument that is expecting an array
Pass PETSC_NULL_ENUM when argument returns an enum instead of

This requires some changes to user FORTRAN code

Pass PETSC_NULL_XXX_ARRAY when passing a NULL as an argument that is expecting an array
Pass PETSC_NULL_ENUM when argument returns an enum instead of PETSC_NULL_INTEGER
Pass arrays (and not scalar values) when the argument is expecting an array; this means replace, for example, the argument v with [v]
Use PetscObjectIsNull(obj) to check if the object is NULL, instead of obj == PETSC_NULL_XXX

The compiler will now automatically prevent you from using the wrong argument type for the first three bullets above
This will also require an update sowing with the new sowing branch this MR is using.

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