New Ctxcal Flat file Option (#5338)

* Added ability to use new monthly flat files for ctxcal

* Added error handling if we can't find the monthly flat file

* Added initial test for new parameter

* Finalized ctxcal test

* Addressed changelog merge conflicts

* Updated history entry in ctxcal

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Co-authored-by: Kelvin Rodriguez <krodriguez@usgs.gov>