refactor: use model class for files and attributesfeat: can use numpy datetime arrays as input time variablefeat: calculator for height differences between ellipsoidsfeat: can use argparse prefix files to define command line argumentsfeat: added generic list from Apache http serverfix: netCDF4 cases where there is no mask on constituent filesfix: USAP now requires capchasfix: pole case in stereographic area scale calculationfix: usegetncattrto get attributes from netCDF4 files to prevent deprecation errorsci: install proj from source for cartopy dependencytest: NSIDC no longer requires authentication headerstest: added test for model definition files