NF - central longitude for stereographic CRS

Allows the user to specify the central longitude for stereographic
projections (Stereographic, NorthPolarStereo, SouthPolarStereo).
Merge pull request #188 from ajdawson/stereographic-central-longitude

Added central longitude for stereographic CRS
Correct invalid NE dataset name in docstring.
Merge pull request #190 from ajdawson/feature-docstring

Correct invalid NE dataset name in docstring.