Pytmd

Release includes various fixes and improvements to the documentation.

- docs: slimmer environment file for prevent overutilization when building on readthedocs
- docs: updated structure of documentation
- fix: ocean pole tide test read
- fix: verify that input data is a masked array in bilinear interpolate
- fix: masking of netCDF4 ATLAS variables
- fix: place some imports behind try/except to include tools import

* fix: improvements for compact ATLAS reads
* feat: added ESR netCDF4 formats to list of model types
* feat: added options to apply flexure to computational programs
* feat: added Greenland 1km model (Gr1kmTM) to list of models
* feat: added field_mapping options to netCDF4 and HDF5 reads
* refactor: changed keyword arguments to camel case
* refactor: use pypi versions of ICESat-2 and ATM1b tools
* docs: add paragraph about delta times

fix: extrapolate for equirectangular case to address 87
feat: added global Empirical Ocean Tide model (82)
feat: added Arc2kmTM to list of available models
feat: option for changing time standard in file programs
feat: ICESat and Operation IceBridge updates
refactor: slim down requirements.txt

- refactor: using python logging for handling verbose output (71)
- fix: Operation IceBridge functions for geotrajectory outputs (73)
- feat: try to get grid mapping attributes from netCDF4 and HDF5 (74)
- refactor: adjust longitude convention based on model longitude (76)
- feat: add TPXO9-atlas-v5 to standard models (77)
- feat: add tide program for ATL10 freeboard files (78)
- feat: added function to calculate a tidal time series (80)

* 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

conda and docker releases
add new Gr1km-v2 1km Greenland model from ESR
add option for extrapolation cutoff in kilometers
add warning for tide models being entered as string to address
bug fixes for nearest-neighbors and bilinear interpolations
documentation updates