Automatic tick placement on Cartopy orthographic projections

[clairefio]

It would be useful to have a function that allows users to specify which ticks they want labeled on which axes spine (top, bottom, left, right) when labeling an orthographic projection.

(See GeoCAT-examples PR #145)

[kafitzgerald]

We should probably look into this again. Especially since that example needs some work anyway NCAR/geocat-examples#566

[kafitzgerald]

Gridliner labels should do the trick for this.

[kafitzgerald]

Some of this has been addressed in Cartopy (see the link above), but it's still not possible to easily specify specific labels to include decoupled from the grid lines. We might watch this open issue SciTools/cartopy#2005 and/or contribute if we have time.