Expand BBS data to include zeros when a species was not seen.

BBS data is distributed by the USGS in a format that only includes data on species of birds that were actually seen. If a species was not seen on a route in given year, it is simply not listed. That is, instead of zeros to indicate "that species of bird was not seen" there is nothing. Also, if a route was not survyed in given year, instead of NA there is simply no rows of data for that route for that year This function adds back those empty rows so that zeros occur appropriately in the data. HOWEVER, the issue of routes not being done in certain years has NOT been addressed yet.

expand_BBS_routes(BBSdata)

Arguments

BBSdata	a dataframe of BBS bird counts