Relax type restriction on ParameterSet#<<

[Mauricio Bedoya]

The type restriction on the method ParameterSet#<<(Array(DihedralType)) should be changed to Enumerable(DihedralType), otherwise Array::View(DihedralType) (returned by ParameterSet#dihedrals) doesn't work. This will require to transform the argument to an array via #to_a before adding it to the internal array of dihedrals.