Creates a map of monitoring stations in a given ws_monitor object. Individual
monitor timeseries are reduced to a single value by applying the function
passed in as slice to the entire timeseries of each monitor with
na.rm=TRUE. These values are then plotted over a map of the United
States. Any additional arguments specified in '...' are passed on to the
points() function.
If slice is an integer, it will be used as an index to pull out a
single timestep.
If slice is a function (not a function name) it will be used with
argument na.rm=TRUE to collapse the time dimension. Thus, any user
defined functions passed in as slice must accept na.rm as a
parameter.