[Fix] Pressure coefficients from external source

[FlorianDeconinck]

Fix passing down ak/bk for pressure coefficients when they are available from an outside source (online model case) and given direct to MetricTerms.__init__

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

[fmalatino]

lgtm