Implementing Cutoff protocol, related synchronization communication tags, and general Improvements to protocol stability

[ba2tro]

Based on #114 and continued from #107

Besides the documentation work from #107 , the following changes are introduced to add decoherence to the protocols

• the latest guid associated with slot is stored in a vector field in Register, earlier there was no way to access a slot's tag information just from the slot, this helps to implement the iscoherent function which just takes a slot and checks whether its coherent or not
• Another vector field added to Register called retention_times which allows the user to provide individual retention time for each slot which is set to 5.0 time units by default
• iscoherent function which takes a slot and returns whether its coherent or not. It also takes a buffer_time parameter, so that a swapper can choose slots that would be coherent for, say 2.0 units of time, and the messages passing is completed before the slot gets decoherent
• EntanglerProt and more specifically findfreeslot has been modified to return already assigned slots that have decohered after it untag!s and traceouts them, this way we don't need a separate protocol to free the decoherent slots and make more mess. This also removes the need for the margin parameter in EntanglerProt that we were passing to findfreeslot and greatly simplifies the function body

This deals with the deadlocks and the plots from the grid example look pretty cool:

grid_sim6x6hv.mp4

[ba2tro]

I see there's work going on in qobase, the docs are building now(other than the random xvfb errors that happen now and then).There was an @ref that doesn't have a doctrine in qsymbolics

[Krastanov]

you are getting these warnings because #143 is in progress. Do not worry about them.

[ba2tro]

@Krastanov this is ready to be merged

[Krastanov]

docs and changelog and version bump

[Krastanov]

Woohoo! Thanks for taking care of this and shepherding it through the review process. I made a few changes, some related to minor stylistic documentation personal preferences, some related to the stability of the tests that were failing. Please check them out to make sure I did not miss anything.

I made a related issue #147 for a few minor things that would be nice to fix, but were not blocking.

[ba2tro]

Thanks a lot for pushing it through!