Make it possible to generate "final" files without the melt onset field(s)

[sc0tts]

Calculation of the melt_onset_day requires the code to generate complete Northern Hemisphere [NH] daily fields for all days from the start of the melt onset period (day-of-year 60) until the date being calculated.

This introduces a requirement that mid/late summer NH fields generate a lot of data, which makes it cumbersome to regression test those dates or even work on examining the non-melt dependent algorithms.

Recommend implementing an override so that the melt onset field can be explicitly omitted from output fields.