backwards-compatible handling of WEST equilibria and MEPHIT

[pazathoth]

The suggested change allows three use cases for field_divB0.f90:

1. The “default” use case of other versions of this file, were a standard gfile is read according to field_divB0.inp, was broken because tetra_grid_settings_mod was referenced in field.
2. After field_divB0.inp is read, settings can be modified; this is now done in load_tetra_grid_inp instead of field.
3. After field_divB0.inp is read, immediately reading the gfile can be skipped. This is necessary for MEPHIT, which modifies gfile data conditionally.

[krystophny]

@pazathoth the change breaks tests with error message

GORILLA - Guiding-center ORbit Integration with Local Linearization Approach
At line 68 of file /Users/runner/work/GORILLA/GORILLA/SRC/field_divB0.f90 (unit = 1738, file = 'field_divB0.inp')
Fortran runtime error: End of file
 

 Tetrahedronal Grid: Loaded input data from tetra_grid.inp
Error termination. Backtrace:
#0  0x10b3ebaae
#1  0x10b3ec7a5
#2  0x10b3ed32b
#3  0x10b6[31](https://github.com/itpplasma/GORILLA/actions/runs/5000498171/jobs/8957976062?pr=11#step:6:32)fb3
#4  0x10b62b31d
#5  0x10b62c0fa
#6  0x10aa16ac2
#7  0x10a9efb9c
#8  0x10a9ea72a
#9  0x10aaaf3b8
Error: Process completed with exit code 2.

pls check

[krystophny]

... and now there's some module problem with input_files

[krystophny]

Converted to draft until we have an integrated solution with libneo.