used 'delta' in Movers.BaseClasses.Characteristics.Power for handling low rpm's instead of a fixed constant to be more consistent with the rest of the implementation of movers
revised implementation of scaling laws in 'efficiency' similar as to previously done in 'power'
set defaults for new parameters of function efficiency() in ControlledFlowMachine.mo:
delta=0, r_N=1, this should be backwards-compatible
improved documentation of efficiency function
Merge pull request #215 from Mathadon/Mathadon-develop

Corrected scaling law implementation of efficiency() similar to power()