Version bump and package 3D utils.
Add some methods to find polymers.

We can find polymers by looking for looking for things which are part of a
different asymmetric unit and then within each we look ones where the
auth_mon_id, as in the author assigned monomer is '?'. I think the '?' will
only occur when the unit was not observed in the structure.
Use a subclass of Table for Polymers.

The Table class has some useful methods. So I'd like to keep using them for
polymers. Thus I made a subclass of Table for Polymer. In addition this
subclass will be useful for slicing the table.