fetchSDA_spatial/SDA_spatialQuery: add support for point/line mapunit and feature point/line geometry

[brownag]

fetchSDA_spatial()

• Allow for by.col to take the value "featkey" for querying point and line features by feature key
• Allow for geom.src to take "mupoint", "muline", "featpoint". "featline"

SDA_spatialQuery()

• Allow argument what to work for the above geom.src options when db="SSURGO"

[brownag]

This issue has been addressed in #345 and #346

I knew it was rare, but didn't quite realize how rare, that it is to have mupoint and muline mapunits... there is only 1 (MI606) and 3 (CO662, WV621, WV622) SSAs that use these constructs, respectively. Luckily it was very simple to add support for these tables as they have essentially the same structure as mupolygon.

In contrast is is very common to have point (featpoint) or line (featline) special features, which is what inspired this issue. SSAs commonly have hundreds of these of various types. Special features are a portion of the SSURGO dataset that does not get nearly enough attention so hopefully making them accessible by queries on featkey (via fetchSDA_spatial()) OR by spatial extent (via SDA_spatialQuery()) will be useful. I think these data points and lines could be especially useful for training models and other types of validation of mapunit concepts with respect to minor components, hydric soils, misc. areas, etc. It would be very interesting to see the "nationalmusym" concept extended to point and line features to make it easier to query features by concept (e.g. all "wet spots" in the state of California), rather than having to use SSA-specific featkey. But harmonizing different feature keys would require fairly substantial changes to the NASIS data model to be able to manage and correlate the keys and the symbols used for each SSA.