Fix intended behavior of demos at instantiation for RegistrationFactory

[alexlee-gk]

Currently, the demos provided at instantiation of TpsRpmRegistrationFactory, TpsRpmBijRegistrationFactory are merely used to iterate over for batch_register and batch_cost. However, the intended behavior is for the RegistrationFactory to only allow to do register and cost with one of the demos provided at instantiation.

Once this behavior is implemented, the RegistrationFactory should do all necessary pre-computation at instantiation for efficiency reasons (i.e. the SolverFactory should create a Solver at this point instead of every time tps_rpm or tps_rpm_bij is called).