Fix to muscles to use initial states provided

Specifically, fixes a bug in the equilibrium solution of Millard and Thelen muscles, where the initial activation and fiber-length values (for solving for equilibrium ) were always coming from the default values. This was unnecessary, because unless specified otherwise, the state automatically contains the default values. When otherwise specified, e.g. from an initial states file, we want the file values to be used and that is how the generic behavior works. This fixes issue #247.

Note, in #247 there is no problem if "Solve equilibrium for actuator states" is unchecked.