Code cleanup

[egonw]

Hi @johnmay, assuming I get that minor CDK grant, I need to be prepared to get started. So, I'm going to get some routine back. How beter than with code clean up.

This small patch is just to get some feedback. I want to remove all remaining hard-coded dependencies on the cdk-data implementation. In doing so, I also moved one test class into the extra module itself. There was a technical reason, being the dependency on abstract test class. Are you happy with moving test class that do not have that dependency into the main module, because then some will be, others will not?

Another perhaps controversial thing is that it copies code from the ElectronContainer and ChemObject classes, to provide the full IElectronContainer API.

Let met me know what you think. This PR is also about the discussion and happy if you cherrypick and/or tell me to extract bits and exclude other bits.

[johnmay]

All looks good, I didn't even know about the "Association" class. From the doc I think a more sane way for storing this is with bond order (Dative/Hydrogen) etc. This then integrates nicely with existing formats (e.g. V3000 can store hydrogen bonds).