fix bug in matching by first column

[Gregory Jefferis]

This change in #467 introduced a subtle bug:

nat/R/neuronlist.R

Line 301 in 49d578d

 matching_rows_fc=intersect(nn, value[,1]) 

If the first column of the incoming dataframe is numeric then intersect(nn, value[,1]) will be numeric as well, which will cause indexing to fail.