refactor: use model class for files and attributes
feat: can use numpy datetime arrays as input time variable
feat: calculator for height differences between ellipsoids
feat: can use argparse prefix files to define command line arguments
feat: added generic list from Apache http server
fix: netCDF4 cases where there is no mask on constituent files
fix: USAP now requires capchas
fix: pole case in stereographic area scale calculation
fix: use getncattr to get attributes from netCDF4 files to prevent deprecation errors
ci: install proj from source for cartopy dependency
test: NSIDC no longer requires authentication headers
test: added test for model definition files