[IMP-dev] This week in IMP

- There is a IMP::Rotation2D

- You can now extract fixed Euler ZYZ angles from a IMP::Rotation3D. This code can serve as a template for extracting other Euler angle conventions if they are needed.

- IMP::algebra::rotation_between_two_vectors() changed to IMP::algebra::rotation_taking_first_to_second() to make it clear which direction the rotation goes.

- IMP::Decorator derived objects are now convertible to bool. So code like if (d) { blah } works.

- IMP::Decorator::cast now returns a null decorator (\c DecoratorName()) rather than throwing an exception. This might make it actually useful. The previous method was entirely unused.

- New IMP::statistics module for statistical methods (e.g. clustering).