ILVIS2 datasets: support ITRF transformation of all lat/lon/elev fields

[trey-stafford]

The ILVIS2 data have multiple sets of lat/lon/elev fields.

Both v1 and v2 have:

• ZG = ELEVATION_LOW
• GLON = LONGITUDE_LOW
• GLAT = LATITUDE_LOW
• HLON = LONGITUDE_HIGH
• HLAT = LATITUDE_HIGH
• ZH = ELEVATION_HIGH

and v1 also has:

• CLON = LONGITUDE_CENTROID
• CLAT = LATITUDE_CENTROID
• ZC = ELEVATION_CENTROID

As of #20 , iceflow replicates the behavior of valkyrie and uses CLAT, CLON, and ZC as the lat/lon/elev fields and these are the only ones that get ITRF transformed.

We should probably support tranforming all sets of lat/lon/elev and also allow users to choose which set they want to use for analysis.