Input validation for date arg of npn_download_geospatial()

[Aariq]

Addresses #97

• Allows a more R-like input for DOY—c(1,5)
• Deprecates character input for DOY—"1,5" with a warning
• Allows Date objects or "YYYY-MM-DD" as inputs

[ezylstra]

Looks good to me. However, I can't test calls to npn_download_geospatial() right now without first changing the base-reqs file (geoserver url http -> https) as Jeff suggested. I wasn't sure whether it makes sense to wait on that or not since Jeff is still working on geoserver stuff.

[Aariq]

Still to do:

This apply coerces numeric inputs back to character, triggering the deprecation warning:

rnpn/R/npn_geoserver.R

Line 499 in 682619c

get_additional_rasters <- function(data) {