Prevent messages persisting between iterations

[ptheywood]

Message lists previously would persist from one iteration to the next, due to comparable behaviour in FLAME GPU 1.

This semi-breaks the intended state machine, but is potentially a useful optimiation in some very edge cases.

This PR (now) implements message lists persistence, through a property on the messageDescription object, e.g.

MessageBruteForce::Description message = model.newMessage("msg");

// Query the persistence, false by default
bool isPersistent = message.getPersistent();

// Set the message list to persist iterations
message.setPersistent(true);

Message lists which are not marked as persistent are set to 0 length and the appropraite flags set to rebuild data structures prior to their next use at the end of each step (i.e. lazy resetting to avoid unneccesary memsets).

• Implement getPersistent
• Implement setPersistnt
• Test getting/setting persistnece in C++
• Test getting/setting persistence in python
• Test persistence in C++
  • Just Bucket so far, others can be deferred to future tests?
• Test persistence in Python
• Docs issue opened Message list persistence FLAMEGPU2-docs#125

Closes #247

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Original PR text for posterity

• Add test(s) which detect messages persisting from one iteration to the next
• Prevent messages persisting from one iteration to the next (via resetting at the end of the iteration)

This is based on 972 due to common test code.

[ptheywood]

Full test suite passes for release with belts under linux:

[----------] Global test environment tear-down
[==========] 1032 tests from 80 test suites ran. (165641 ms total)
[  PASSED  ] 1032 tests.

  YOU HAVE 40 DISABLED TESTS

If any models rely on persistence then this will break them however.

[Robadob]

If any models rely on persistence then this will break them however.

Yeah I don't agree with this change. Add a HostAPI method for users to manually purge message lists, and bin this PR.

[ptheywood]

This is now ready for review, as a PR which makes non-persistent messages default, with opt-in to persistence for each message list.

Adds C++ and python tests for new features, but only checks persistence behaves correctly for brute force messages and bucket messages (in the interest of time, they are not simple tests).

[Robadob]

Haven't tested personally, but looks thorough (you have a Python test which would have been my main concern).