[IMP-dev] Minor changes to xyzdecorator

Here are some minor changes to the XYZDecorator.

It now always creates the keys since we are always using the. This means that the get_coordinate_key(i) method doesn't have to check when getting the keys out.

It also has methods to change whether the particle is optimized or not. These methods are checked as part of the SphereRestraint patch I will submit separately.