Improve handling of units

[jesper-friis]

Consider to add a thin abstraction layer around pint to make it easier to replace it later with another unit package. It could be a new module called tripper.mappings.units. This module could define two classes

• UnitRegistry - wrapper around pint.UnitRegistry
• Quantity - wrapper around pint.Quantity

As suggested by @sygout, an additional benefit with this is that it would allow for not calling pint if the source and destination units are the same.