Relative paths break `submit-pyrosar-gamma-workflow` using `--output-dir`

[truth-quark]

There's a small bug with the submit-pyrosar-gamma-workflow utility. If using --output-dir <some-path> with a relative path, the workflow can break. This is shown in the error logs with an OSError like this:

  File "/g/data/yp75/<userid>/miniforge3/envs/sar-pipeline/lib/python3.12/site-packages/pyroSAR/gamma/error.py", line 71, in gammaErrorHandler
    raise errortype(err_out)
OSError: cannot open output data file: 20260318-13h45m34s-multi/data/temp/S1A__EW___A_20250419T221041/S1A_EW_GRDM_1SSH_20250419T221041_20250419T221148_058831_074A60_9416

This doesn't point to the cause being a relative path. Ideally, users should be prevented or at least warned specifically.

Tasks:

• Update markdown docs, ask users to specify absolute paths
• Update CLI docs to ask users to specify absolute paths
• Determine what happens when reading output_dir var from the .toml config file
• Add defensive code & fail fast if relative paths are used for --output-dir

[caitlinadams]

Started this work at https://github.com/GeoscienceAustralia/sar-pipeline/tree/fix/relativepaths

[truth-quark]

Considering how to alert users, there's a choice of warnings or exceptions.

if not os.path.isabs(output_file):
    # can also fail from CLI arg or `output_dir` in the `.toml` config
    warnings.warn("Using a relative path for --output-dir")

A warning is a lighter approach, but could be buried amongst logged exceptions/traceback should a user overlook the docs (I'd assume this is a likely outcome).

if not os.path.isabs(output_file):
    raise SomeError("Using a relative path for --output-dir...")

Raising an exception has the benefit of probably appearing last in the logs.