use pint for unit conversion

[Joern Ungermann]

The pint package is very useful to interpret unit strings and convert units to one another. We should use pint to determine the conversion factors instead of the very simple approach currently used.

Some special handling might be required for geopotential altitude (e.g. m^2s^-2 → km)

[Joern Ungermann]

Partially adressed by PR #660

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Should be extended to force/allow specification of units in plots and automate the conversion.

[Reimar Bauer]

Force specification of pint-parseable units in plot styles and netcdf files

Implemented mechanism for automatically converting units to the proper type. * Enforcing parseable units attribute being present in data files * adjustements to units of demodata * Updating of all plotting styles.

Fix issue #422

→ <<cset b28f25a527db>>