Switch to non-weighted stats, bugfixes, and ngen compatability fixes

Combination of fixes and improvements to work with ngen. These changes allow the generated files to be used with the current version of ngen.

Additions

weighted_sum and weighted_mean stats changed to sum and mean
Fixes all ids values being truncated to one character
Changed the way time units are stored--the previous way is actually what is recommended by Unidata, so this may be changed back, but this is what ngen currently supports.
Time is set to an unlimited dimension
Time values are now a series per catchment--varying time values per catchment are not yet supported but we wanted to structure the file format to possibly support it in the future.

Used this version of the script to generate a HUC01 forcing data file from the new 09/2022 hydrofabric, and ran it in ngen.

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