Improve ODB2 format support

[Scott Wales (ScottWales)]

Refactor ODB2 support

Fixes #2102

• metplus_prep_obs opt file odb2 will read ODB2 files from an arbitrary directory set using $CUSTOM_ODB_PATTERN. This pattern can use strftime placeholders and wildcard globs, and will automatically decompress the files if it detects a matching extension.
• Class PrepODB2 is an interface for sites to fill in their own ODB2 directory patterns, see the example for BOM data at NCI in odb2/bom.py. BOM data on internal systems will use this interface as well in a restricted repo.
• Add support for more ODB2 observation types
• Correct the levels of surface data, now at Z0 instead of P0, so they get picked up by verpy correctly

At NCI-Gadi site observations can now be sourced from either a BOM operational system with

METPLUS_OBS_SYSTEM = "access_g4"

or from ODB2 files on disk with

METPLUS_OBS_SYSTEM = "custom"
CUSTOM_ODB_PATTERN = "/path/%Y/%m/%Y%m%dT%H%MZ/*.odb.gz"

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