Update fock drudge for more flexible customization
Update fock drudge for more flexible customization

Now in the drudge for Fock spaces, the contractor and comparator
required by the Wick class, as well as the vector colour function for
the base class, are all generated on-the-fly from properties of the
class.  In this way, subclasses can have more flexible customization of
the normal-ordering behaviour.

tschijnmo/drudge,tschijnmo/drudge,tschijnmo/drudge