Clear model function for atlas builder

[StephanieHaeger]

The atlas builder runs svf registration in a for loop and uses model properties from the previous registration for the current registration. A function that clears the model beforehand is needed.

[marcniethammer]

Should be possible now. For the simple registration interface, one can now do the following:

import pyreg.simple_interface as si reg = si.RegisterImagePair() ...

reg.set_model_parameters_to_zero()

This should be done with care though, because it will set all model parameters to zero.