Provide units field to Plot routines

[Joern Ungermann]

Currently, the data is automagically provided to the plotting routines. It is not clear, however, what units are provided. It is thus easy to provide the wron value in the NetCDF file without noticing (e.g. Pa against hPa). The plotting driver should read out the units field and provide the content to the plotting routines such that they can assert the proper units (for debugging!) and/or provide conversions. This would be a first step to support (semi-automatic) conversion in user-space using some udunits package or such.

The issue is really annoying as vertical plots require Pa as vertical axis, but some files contain hPa others Pa and the corresponding plotting routines are not really usable for the wrong units.

[Reimar Bauer]

Read out units attribute and provide to plotting styles

The plot driver accesses the netCDF file when acquiring the data. it is straightforward to read out the units attribute and provide it to the plotting styles base classes for further use.

Resolve issue #219

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