Add start/stop model regression testing package

[biljanaorescanin]

Add regression test framework for model runs

This commit introduces a new regression testing framework under
GEOSldas_App/util/postproc/regression .

It includes:

• README.md with detailed instructions for running regression tests
• start_stop_model.sh script to manage model execution for tests
• HISTORY_1d.rc and HISTORY_2d.rc templates to ensure consistent
  output (for both CS and EASE grids)
• Updates to ldas_setup to enable regression test configuration

Notes:

1. At this stage, the framework supports model-only regression runs
   (not assimilation).
2. Depending on tile types, the model may run land-only (lfs, lnd) or land+landice (lfs, lnd, glc) collections. The regression now detects this automatically.
3. This work addresses one of the items discussed in:
   revisit LDAS nightly tests GEOSldas#250
4. The design follows the approach used in GEOS GCM regression testing.
   Since GEOSldas does not support 2D layout tests, we use a 1D-invariant
   approach analogous to:
   https://github.com/GEOS-ESM/GEOSgcm_App/wiki/Regress-Script-Flow-Chart
5. This implementation is a proposed starting point. The approach can
   be modified or the PR closed if it is not aligned with how we wish to
   support regression testing in GEOSldas.

[biljanaorescanin]

Testing summary:
Trivially zero diff for GEOSldas nightly regression tests.

Regress runs tested:
CF90 tile type 100 20 (collections lnd, lfs and glc)
CF90 tile type 100 (collections lnd and lfs )
EASE M36 tile type 100 20
EASE M36 tile type 100