Incorrect units should cause an error

[wattrg]

Currently if incorrect units are specified for Quantities, the code will proceed without warning/error, however it will output nonsense to the output files. For example, adding
AddQuantity(name="mu", unit="H/m", comment="magnetic permeability")
to Dynamics.R won't cause any errors, but the values of mu which are outputted won't be correct because "H" is not a valid unit.

Instead a warning or error should be given saying that an invalid unit has been specified.