fix the units on FHD reference frequencies

[bhazelton]

Description

The units on FHD catalogs are always in MHz but were being read in as if they were in Hz. This fixes that.

Motivation and Context

fixes #213

Checklists:

Bug Fix Checklist:

• I have read the contribution guide.
• My code follows the code style of this project.
• My fix includes a new test that breaks as a result of the bug (if possible).
• My change includes a breaking change
  • My change includes backwards compatibility and deprecation warnings (if possible).
• I have updated the CHANGELOG.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (3620163) 100.00% compared to head (eb2c730) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #224   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            5         5           
  Lines         2115      2115           
  Branches       600       600           
=========================================
  Hits          2115      2115           

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[nicelmh]

Looks good to me!