Complete implementation of optional arguments / active variables in capgen

[climbfuji]

Description

PR #552 introduces optional arguments in ccpp_prebuild.py based on requirements described in https://github.com/NCAR/ccpp-framework/issues/526. The PR discussion led to a set of requirements that go further than what is described in #526: https://github.com/NCAR/ccpp-framework/pull/552#discussion_r1603198387. ccpp_prebuild.py implements those in #552 in the prebuild layer on top of the capgen layer.

Solution

Some of that functionality is not yet available in capgen.py and needs to be added there (with tests).

Alternatives (optional)

n/a

Related to (optional)

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