* Initial commit for CollectSignaturesFlow, some tests and associated documentation via a new "Flow Library" section of the docsite.
* Refactored the TwoPartyDealFlow to use the CollectSignaturesFlow.
* Added the subclassed CollectsigsFlow to the trader demo, whitelisted it and added a flow initiator for the responder.
* Minor edits to progress tracker.
* Amended as per Rick's comments.
* Generalised this flow, so it now works if more than one signatures have been collected, initially.
* Minor edits to the IRS demo so it uses the CollectSignaturesFlow.
* For debugging purposes...
* Adding CollectsigsFlow support to SIMM Demo.
* Removing debug logging.
* Amended top level comment: transactions can only have one notary.
* Added TODOs as checkTransaction logic is absent.
* Addressed Mike's review comments.
* Minor edit to flow-library docs.
* Updated flow based on Mike's review comments.
* Added two usage examples and updated the tests.
* Made changes to accommodate new CollectSignaturesFlow approach.
* Made changes to SIMM demo to accommodate new CollectSignaturesFlow approach.
* Added abstract check proposal method to two party deal flow.
* Added missing TODOs.
* Addressed Sham's comments.
* Rebased to M11.
