Added backrub rotation angle customization to BackrubMover
BackrubMover now has the following new data members:
bool custom_angle_ // true if backrub rotation angle is customizable
bool custom_angle() // gets custom_angle_
void set_custom_angle(bool) // sets custom_angle_
Real next_angle_ // the rotation angle for the next backrub move
Real next_angle() // gets the rotation angle for the next backrub move
set_next_angle(Real) // sets the rotation angle for the next backrub move
custom_angle_ is set to false by default to preserve normal backrub behavior