I cleaned up the bond decorator code. Among other things I removed the
length parameter with the expectation that I will add another similar
set of decorators for custom connectors (not quite sure what to call
them). These will represent arbitrary length attachments between
particles. I also added some test code for the bond decorators.
The connectors will have a length and stiffness parameter and will be
handled all together by a restraint (along the lines of what was
discussed with the non-bonded list). Normal inter-atomic bonds could
be handled by generating the appropriate lengths from the names of the
atoms involved in each atomic bond and adding the connector information.