Ungauged bias correction functions

[rileyhales]

Todo:

• [ ] upload modeled FDCs to aws
• [ ] upload observed scalar FDCs to aws
• [ ] upload saber table to aws
• [ ] need function in data.py and _download_decorators.py to retrieve fdc data
• [ ] need function to perform sfdc mapping copy/paste from saber-hbc repository
• [ ] need function to perform lookup of gauge sfdc to use for a given linkno
• [ ] need function to check environment for saber table path
• [ ] need function to download copy of assign_table from aws and/or to check the environment variable for it (see data.py starting at line 205).