Change bind() to be predictable

[nicholascar]

g.bind() has two input parameters, override & replace which are confusing to understand and, even according to the describe behaviour may be buggy (e.g. adding a "p" to a duplicate prefix in a non-deterministic selection of the duplicates).

There also seem to be down-stream bugs or different handling in Turtle & RDF serializers which handle duplicate prefixes differently.

Propose:

• replace g.bind(override=True, replace=False) with g.bind(force=False) where:
  • force=False: an error is raised if an existing prefix is re-bound
  • force=True: the new prefix is bound and the previously declared duplicate is un-bound

This should remove any second-guessing from serializers.