Add function to find local extrema and plot separately

[kgoebber]

There is a Issue #1234 and PR #1273 that is a first attempt to find and plot local extremes for gridded data based on an example from the Python gallery. This new request is to get that back on the radar screen and to disaggregate the finding and plotting of local extrema.

In the plotting function it would be good to include potential options for including an outline and choosing specific colors, depending on how the function is structured

Tasks:

• function to find local extrema (location calc tools?)
• function to plot extrema (location plots)
• tests for both functions