Add channel support to ioda-stats

[CoryMartin-NOAA]

Now that ioda-stats works for specified domains, and regular gridding, it needs to be modified to handle observations that use channels (i.e. satellite radiances). I believe that IODA will return an array like so:

[ ch1_loc1 ch1_loc2 ch1_loc3 ch2_loc1 ch2_loc2 ch2_loc3 ]

which will require some slicing of the data when both computing values and writing to file

[CoryMartin-NOAA]

https://github.com/NOAA-EMC/DA-utils/tree/feature/channel-stats will be where the work is performed