Ability to generate planar metric terms and addition of `test_eta.py` unit test

[fmalatino]

Description A conditional statement has been added to the __init__ method of the MetricTerms class to allow for generating surface level metric terms (no hybrid pressure values). Initialization of such a MetricTerms object will now contain zero-valued hybrid pressures.

The unit test test_eta.py from pace/tests/main/grid has now been moved into ndsl/tests/grid along with a python module for generating the test eta level containing files.

This PR must be reviewed and merged after PR 92 is merged in pace

How Has This Been Tested? Tested via the tests contained within the tests directory of NDSL, and in the tests of the dependent repositories

Pace tests will fail as the changes from PR 92 in Pace are necessary for passing.

Checklist:

• [x] My code follows the style guidelines of this project
• [x] 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
• [x] My changes generate no new warnings
• [ ] Any dependent changes have been merged and published in downstream modules
• [x] New check tests, if applicable, are included

[fmalatino]

Given that metric terms, and the grid generation methods as a whole are a part of NDSL, does it make sense to move the unit test tests/main/grid/test_eta.py in Pace to NDSL?

[FlorianDeconinck]

Given that metric terms, and the grid generation methods as a whole are a part of NDSL, does it make sense to move the unit test tests/main/grid/test_eta.py in Pace to NDSL?

Yes, please pull it down. We decided that the FV grid would come preset with NDSL, let's commit fully to it.