openfoam: add 'precision' easyconfig param (DP/SP/SPDP)

[hmeiland]

What

Adds a custom precision easyconfig parameter to the OpenFOAM easyblock, so single-precision (SP) and mixed-precision (SPDP) OpenFOAM modules can be built directly from an easyconfig instead of a manual WM_PRECISION_OPTION override.

'precision': ['DP', "Floating-point precision: DP, SP, or SPDP", CUSTOM],

Why

OpenFOAM's etc/bashrc sets WM_PRECISION_OPTION=DP unconditionally, so simply exporting the env var is clobbered the moment Allwmake sources bashrc. The sanctioned override point is etc/prefs.sh (sourced after the DP default and before WM_OPTIONS is computed).

For non-DP builds the easyblock now:

• writes WM_PRECISION_OPTION=<prec> into the install's etc/prefs.sh (idempotent — only if not already present),
• sets the build-time env var,
• threads the precision into the platform subdir name (linux64Gcc<PREC>Int32Opt),
• exports WM_PRECISION_OPTION in the generated module.

The concrete motivation is the petsc4Foam (OpenFOAM external-solver) adapter linked against a single-precision PETSc (--with-precision=single), which requires PetscScalar == Foam::solveScalar == float, i.e. an SP OpenFOAM.

Backward compatibility

The parameter defaults to 'DP', which reproduces the previous behaviour exactly:

• self.wm_precision != 'DP' is false → no prefs.sh is written,
• the subdir string resolves to the same ...DP... as the prior hardcoded literal,
• the new WM_PRECISION_OPTION=DP env/module export matches what OpenFOAM's own bashrc already sets.

So existing OpenFOAM easyconfigs (which never mention precision) build unchanged.

Testing

• python -m py_compile clean.
• Used in practice to build an SP OpenFOAM v2506 (foss/2025a) module on AMD gfx1101, consumed by a single-precision petsc4Foam adapter; DP builds verified to produce byte-identical psubdir/module env vs. the stock easyblock.

Draft: opening for early feedback on the approach (custom param + prefs.sh write) before adding an SP easyconfig that exercises it.