Improvements to the Abbreviation algorithm, options, and functionality.

• We can now generate more (and better) abbreviations than before, for example. Et3P and Ph2
• Rather than some true/false flags, there is now some enum options to tune how things work: factory.with(Abbreviations.Option.ALLOW_SINGLETON); factory.without(Abbreviations.Option.AUTO_CONTRACT_TERMINAL);
• When abbreviations are applied has be tweaked, previously if <40% of the atoms were in the contraction it was applied. Now it does <50% but splits out ring atoms. This means if there is a single ring in a structure or two rings of the same sizes it does not get contracted.
• It is also now possible to provide an indication that atoms should be keep together and not have an abbreviation split them. This is most useful to substructure highlighting.