Replacing init_gravity stencil with use of set_value from NDSL

[fmalatino]

Description
Include a summary of the change and which issue is fixed. Please also include
relevant motivation and context. List any dependencies that are required for
this change.

Fixes # (issue)
If this is a hotfix to a released version, please specify it

How Has This Been Tested?
Please describe the tests that you ran to verify your changes. Please also note
any relevant details for your test configuration (e.g. compiler, OS). Include
enough information so someone can reproduce your tests.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules
• New check tests, if applicable, are included
• Targeted model if this changed was triggered by a model need/shortcoming

[jjuyeonkim]

I like how the single _init_gravity can be used for both grav_var and grav_var_h now.