[IMP-dev] Minor cleanup to bonds

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.