Units are not user-definable and are not used consistently

[heikoklein]

Describe the bug
In pyaerocom up to current versions, units are hard-coded in variables.ini and are not user-definable. These units are used for model-input and thus the geojson contourplots on aeroval.
For collocation, the units from the measurements are used.

This makes can make units inconsistent, i.e. when plotting stations on top of contours. It might also make units inconsistent when showing All measurments at once, i.e. when different measurement-networks use different units.

To Reproduce
Steps to reproduce the behavior:

1. change units in variables.ini
2. plot contours
3. show contours with obs under Maps in aeroval

Expected behavior

• The units from variables.ini should be used for both contours and measurements.
• A modeller should be able to provide a variables.ini file overwriting the default values

Additional context

• Aeroval should warn when units are changing: https://github.com/metno/AeroToolsIssues/issues/62
• Color-bars and units are closely related, maybe the same user-config should be used for the color-bars and the units. see make the colour bars for the contours and the map user defined #1221