Fix ERA5 native6 fix to handle single monthly-averaged NetCDF files.

[rbeucher]

Description

Modifies the get_frequency function in ERA5 native6 fix to handle case where monthly-averaged data are stored in single NetCDF file (one file per month) and have thus a time dimension length of 1.

Closes #2511

Link to documentation:

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[rbeucher]

Hi @schlunma, @valeriupredoi,

Could someone please take a moment to review this for me? Thanks!

R