Fix unit system example with solve

The documentation has an example on how to use solve with the unitsystem
module, but there was difficulties due to Dummy argument. As a (temporary)
solution the system is solved before replacing with units.
As a side effect I had to allow non-integer powers for dimensions, since they
may appear in intermediate steps.
Add methodto Dimension to test if all powers are integers
Make UnitSystem callable

The __call__ is a wrapper towards the conversion method of a unit into the base
units of the system.