date handling enhancement for ply2erddap()

[bbest]

A follow-up enhancement to #57 would be to:

1. Query the ERDDAP server for the list of dates.
2. Build index for applicable dates to year, month parameters (and consider designing this as a date_beg, date_end range)
3. If multiple dates, then handle raster brick with some default averaging (method = mean), with option to use a different method like sd() (standard deviation); This can be handled with simple raster algebra, that should allow for na.rm = T as an argument; eg sum(raster_brick, na.rm=T).