Simplify unit conversion

[Joern Ungermann]

Converting data fields in the plotting routine is a manual and rather verbose affair.

One should simplify this by taking units into account when "requesting" variables or offer a much simplified method in the base class for the conversion along the lines of

self.convert("air_pressure", "Pa")

[Joern Ungermann]

A first draft has been prepared in 647b8ba6 but it is obviously not complete. Further work should use the shim for all data access, and ideally a lightweight, external library could be leveraged.

[Reimar Bauer]

@joernu76 should we close this and write a new issue for the next step?

[Joern Ungermann]

Fine for me. The next step would be leveraging metpy/pint.

[Reimar Bauer]

partly implemented by #647b8ba6