Add draft WCS vignette

[brownag]

based on @dylanbeaudette's tutorial http://ncss-tech.github.io/AQP/soilDB/WCS-demonstration-01.html
for #263

TODO:

• figure margins
• other aesthetics (maintain low dependencies; no rasterVis, lattice or color palette packages)
• harmonize approaches and specification of coordinate reference system? Consider using "OGC:CRS84" and not using bare integer codes e.g. "EPSG:5070" rather than 5070
• link out to get_SDA*() function specific vignette for better detail on tabular data acquisition
• use hcl.colors() from {grDevices} instead of viridis::viridis()
• use get_SDV_legend_elements() for interpretation thematic map colors and labels

A couple things I noticed... not causing issues internally, but may cause confusion for users. Dylan mentioned ordered factor issue recently and now I experienced it with some of the examples i.e. Concrete Corrosion.

• investigate ordered factor level preservation in RAT?
• investigate join of character values to RAT using levels<- v.s. replacement of RAT

[brownag]

• link out to get_SDA*() function specific vignette for better detail on tabular data acquisition

will be addressed in future part of #157

[brownag]

• Opting to not use hcl.colors().
• Investigated ordered factor level preservation and it is non-trivial to implement, though within in-memory terra objects it is technically feasible

[brownag]

For some reason this PR is not updating. There are 2 more recent commits on this branch, for a total of 5, including a merge commit updating w/ recent changes on masterEverything seems to be in sync now, ready for review

[brownag]

Merging after discussion w/ @dylanbeaudette; this is functional but will be revised further in coming weeks/after new vignettes are added