Add get_SDA_cosurfmorph()

[brownag]

This ports over and generalizes the concepts from (deprecated) sharpshootR queries.

• hillslopeProbability(), geomPosHillProbability(), geomPosMountainProbability() and surfaceShapeProbability()
• These queries were deprecated in sharpshootR when cached versions of these tables started being provided by SoilWeb and series-level summaries were added to fetchOSD() extended data.

There was a general need to expose how these queries work and extend them for other geomorphic data elements (terraces and flats). A single base query is used for all of the source cosurfmorph* tables

This PR adds a single function get_SDA_cosurfmorph() in the "SSURGO On Demand" style. Users can customize the WHERE clause, target tables (cosurfmorphgc, cosurfmorphhpp or cosurfmorphss) and the grouping variable used to calculate proportions (default by="compname")

• "cosurfmorphgc" summarizes "geomposmntn", "geomposhill", "geomposflats", "geompostrce"
• "cosurfmorphhpp" summarizes "hillslopeprof"
• "cosurfmorphss" summarizes "shapeacross", "shapedown", and concatenated "surfaceshape"

Remaining TODOs:

• Add areasymbols and mukeys arguments for parity with other SSURGO on Demand queries
• (not in this PR) Add custom WHERE clause to other SSURGO on Demand queries (to beused when areasymbols/mukeys not specified)
• toggle STATSGO
• local dsn for e.g. SQLite SSURGO instances