Add example of coordinate transformation, e.g. from UTM Nad83 to UTM Nad27

[mycarta]

This would be very useful.

MINIMAL TO DO LIST:

• Get some test data.
  Ideally a point set with both the source coordinates and transformed coordinates so as to also include a test.

• Get up to speed with pjproj then build a notebook
  I used to do it all the time so I have some old notebooks and .py files to do that but have had no time to catch up with the syntax changes post pyproj 1, see for example here

[mharty3]

@mycarta, you may want to look into the rioxarray package. It has great spatial functionality on top of xarray.

[RichardScottOZ]

Definitely recommended!