function to plot a map of abundance. this was developed for spatio-temporal models in mind
plot_N_map(cur.t, N, Grid, highlight = NULL)
time step to plot
A vector of values to plot (e.g. abundance)
A list of SpatialPolygonsDataFrame (one for each time step) - holding survey unit spatial information
If provided, the rows of Grid[[cur.t]] to specially highlight
A ggplot2 object