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.