Add ability to detect and delete feature classes

[bltern]

It would be very nice, actually it's really necessary, to have the ability to delete feature classes from a File Geodatabase from R via the r-bridge. It's very tedious to have to do this manually. Also to see if it exists already would be useful alone.

[scw]

@bltern thanks for your report. I agree this is useful, and often necessary functionality if you're using R exclusively for managing your resulting datasets. We'll take this into consideration.

[dpavlushko]

arc.delete() and overwrite flag added in 1.0.1.229