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