Clean parsing of optional `config.yaml` data

[abhaasgoyal]

Fixes #234, #239

Merge Message

• Parse config.yaml and fill all optional data before validation. We assume that using keys in config would never lead to KeyError in other modules after validation.
• Correction for validating config schema:
  • Change multiprocessing to multiprocess to match documentation
  • Regex match forwalltime to match single digit numbers
  • Remove Deprecation warning for (?i) flag at the start for mem

[codecov[bot]]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (d1d6280) 58.22% compared to head (2efcd48) 59.13%. Report is 1 commits behind head on main.

Files	Patch %	Lines
benchcab/benchcab.py	0.00%	4 Missing :warning:
tests/test_config.py	97.36%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #238 +/- ## ========================================== + Coverage 58.22% 59.13% +0.90% ========================================== Files 29 29 Lines 2109 2129 +20 ========================================== + Hits 1228 1259 +31 + Misses 881 870 -11 ```

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[ccarouge]

Actually, can we rename read_optional_data? I don't like the use of data here. In the doc, we are using key for the entries in the config file maybe that's the best choice.