Support custom input file paths via command-line flag

[01stamat]

Background

Running GORILLA currently requires all input files (gorilla.inp, tetra_grid.inp, field_divB0.inp, gorilla_plot.inp, preload_for_SYNCH.inp, …) to reside in the working directory from which test_gorilla_main.x is executed, under their fixed conventional names. The filenames are hardcoded across multiple source modules (e.g. gorilla_settings_mod.f90, tetra_grid_settings_mod.f90, field_divB0.f90, …). This makes certain workflows cumbersome and breaks location-independence of the executable.

Proposed change

Add the option to specify the location of input files at execution time, for example:

./test_gorilla_main.x -input /path/to/input/dir
# or
./test_gorilla_main.x --input-dir /path/to/input/dir

or possibly even individual input files living in different directories. Existing behaviour — reading from the current working directory — is kept as the default so that no current workflow breaks.

Scope

• Parse an optional -input <dir> (or equivalent) flag from the command line at startup.
• Pass the resolved path or input files through to every module that opens a named input file.
• Fall back to the current working directory when the flag is absent (fully backward-compatible).
• Update the README / documentation to describe the new flag.