Make unit handling more robust

[henrikjacobsenfys]

If a user (or Parameter dependency expression) changes the unit of a Parameter in a Component, the calculations will give incorrect results.

The reason for this is that the code currently only checks the unit of the Component, not each of the Parameter's in it.

One solution is to check that each Parameter has the correct unit in _prepare_x_for_evaluate, instead of just checking the Component.

Another solution is to not work with .value, but directly with the Parameters