Add accumulation_type to VariableSpec and ComponentSpecParser

[darianboggs]

Types of change(s)

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Trivial change (affects only documentation or cleanup)
• Refactor (no functional changes, no api changes)

Checklist

• Tested this change with a run of GEOSgcm
• Ran the Unit Tests (make tests)

Description

For MAPL3 time averaging, the VariableSpec needs a variable for accumulation_type, and ComponentSpecParser needs to parse accumulutation_type from the ESMF_HConfig. This PR implements these capabilities.