Based on codes from Christy Rollinson and from Max Joseph (http://mbjoseph.github.io/2014/11/08/nlcd.html)
Arguments
- buffer
search radius (meters)
- coords
data frame containing elements 'long' and 'lat'. Currently just supports single point extraction.
- data_dir
directory where input data is located. Can be NUL if con is specified
- con
connection to PEcAn database. Can be NULL if data_dir is specified
- year
which NLCD year to extract. If data_dir not provided, must be one of `2001` or `2011`