Various small enhancements and bug fixes

Enhancements
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* CF/Radial files produced from write_netcdf have a single string_length variable, a field_names attribute, better follow the ARM metadata standard, and the underlying code better handles string arrays.
* The read_sigmet function returns data so that times are monotonically increasing (but times are allowed to repeat)

Bug Fixes
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* non-collected gates in Sigmet files are masked in the returned moment fields.
* The read_mdv function returned radar objects with 1-dimensional latitude, longitude, and altitude data.