Vegplot Transect: Add support for Transect Ground Surface Cover

[brownag]

Add get_vegplot_groundsurface_from_NASIS_db() and get_vegplot_transplantfoliar_from_NASIS_db() from @phytoclast

Following from discussion in the FPAC-NRCS R User Group Teams channel.

• I have tentatively added get_vegplot_BA_snags_from_NASIS_db() but I think it could potentially be combined with existing functions
• Documentation could be cleaned up and made more consistent across all vegplot vegetation data functions
• si argument for converting units could be implemented for other pre-existing functions and columns See Vegetation Data: unit conversion for above-ground measurements #374

[brownag]

Confirmed the existing query in get_vegplot_from_NASIS_db() already includes siteiid, siteobsiid, vegplotid, vegplotname, overstorycancontotalpct, basalareaplottotal, treesnagdensityhard, treesnagdensitysoft which are all 1:1 with vegplot table, so get_vegplot_BA_snags_from_NASIS_db() can safely be omitted from this PR

[brownag]

Also get_vegplot_transpecies_from_NASIS_db() already covers the data elements returned by get_vegplot_transplantfoliar_from_NASIS_db().

The unit conversion portion would be valuable to incorporate--need to look at all the columns that need SI unit conversions to handle it more generally.

[brownag]

get_vegplot_groundsurface_from_NASIS_db() has been added to the vegplot data returned by fetchVegdata() and tested for basic functionality via the soilDBdata snapshot