Add constituent tendency capability

[peverwhee]

Summary Adds constituent tendency array and enables the use of individual constituent tendency variables.

Description In order to facilitate time-splitting, this PR adds/enables the following:

• ccpp_constituent_tendencies: standard name for constituent tendencies array
• standard names of the form "tendency_of_CONSTITUENT" now handled by the framework
  • must also include "constituent = True" in metadata for the tendency

User interface changes?: Yes, but optional User can now use the following in scheme metadata:

• ccpp_constituent_tendencies (standard name for the array of constituent tendencies)
• tendency_of_CONSTITUENT (tendency array for a specific constituent)
• constituent = True (specifies variable will be handled by constituents object)

Testing: test removed: None unit tests: Added doctests to new check_tendency_variables function in host_cap.F90 system tests: Modified advection_test to use tendency variable and tendency array manual testing: Updates run in CAM-SIMA

[climbfuji]

@peverwhee Please re-request a review from all the reviewers once you've addressed the questions/concerns from @mwaxmonsky and @dustinswales (I keep coming back to re-review but then remind myself that there are still comments that need to be addressed). Thank you!

[climbfuji]

@peverwhee Can you please update the branch from develop? There are merge conflicts that need to be resolved. Thanks!

[peverwhee]

@DomHeinzeller done!

I think I'll give @gold2718 a bit more time if he'd still like to review this before merging. Say, by next wednesday (11/6)?