Add declarative attribute plot_units

[kgoebber]

Description Of Changes

This adds a new attribute, plot_units, to help with simple unit conversions for plotting. It is available for all Plot2D() subclasses including ContourPlot(), FilledContourPlot(), ImagePlot(), and BarbPlot().

Two new tests have been added to cover unit conversations for scalar and barb plots.

Running declarative tests fail likely due to running Matplotlib 3.1.2 locally and positions of text on some plots (especially those tests using PlotObs()) change slightly. Don't know if they will fail on Travis, so we'll see.

This is an immediate need for teaching class in just over a week in order to not have to over complicate making simple maps with our common units, which are not the stored SI units in our data files (e.g., Celsius instead of Kelvin and Knots instead of m/s).

Checklist

• Tests added
• Fully documented

[dopplershift]

I can merge as-is if need be, but I think using the accessor for unit-handling would make the code much simpler.

[dopplershift]

I think you should be able to do this as data_subset.metpy.convert_units(self.plot_units). That would eliminate the need to then set the unit attribute below I believe.

[jthielen]

I concur, as this will prevent the need for an immediate refactor after #1304.

With the decision made in #1304, I would definitely recommend that the units of a DataArray should only be handled by the accessor (since the units will soon be part of the variable data inside of the DataArray rather than an attribute on the DataArray).

[dopplershift]

Here too.

[kgoebber]

Well that was a lot cleaner and more robust!

[dopplershift]

Looks like some changes to underlying versions changed our coverage, but unrelated to your changes. I'll open an issue about coverage, but we can merge over.

[dopplershift]

@kgoebber Was there a reason we didn't add such support to PlotObs?

[kgoebber]

Not really, just didn’t focus on that originally, but would be great to have. Can look at it tomorrow and potentially get a PR in.

[kgoebber]

So investigating this morning this would be a bit more involved on two fronts. First, there is not as easy of a conversion mechanism yet for non-vector observation quantities within StationPlot(), second, if reading in surface data from a csv file there will be no units attributes, so there will be a need to bake in some checks and raise errors altering when conversion can't happen as a result of no units as part of the dataframe.

Overall it shouldn't be too hard and I can probably get a PR done by the end of the day, but will involve modifying stationplot.py to make it easier to convert parameter units on the fly and then back that attribute into the declarative PlotObs().

Currently there is the ability to convert the units for vectors (both barbs and arrows) in StationPlot(), but not a parameter. The function doing the converting is _plotting_units(), so my initial thought is to change that to _vector_plotting_units() and craft a new _scalar_plotting_units()`.

[kgoebber]

This PR partially resoled an issue in #1248 .