Allow pressure coeff to be read from ak & bk data

[FlorianDeconinck]

Description
In order to facilitate integration with existing model, we allow the pressure coefficients AK/BK to be read from data. We keep the same checks and give precedence to the data to keep backward compatibility.

How Has This Been Tested?
Tested on GEOS integration. No unit test present but default behavior did not change.

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

[FlorianDeconinck]

PR died because of mpi4py build failing on the setup side of things, will investigate.

[fmalatino]

lgtm