Fix SAYNT

[TheGreatfpmK]

SAYNT currently does not work without including this workaround in stormpy: stormchecker/stormpy#158 which means that in the current state in each release of paynt, SAYNT will not work.

To fix this we should make the belief exploration in SAYNT not be time-based such that multiple threads are no longer needed. Each belief exploration will be just a function call of the belief explorer object in Storm. This will require:

• some update of how exploration is done in Storm
• update the stormpy bindings accordingly
• update SAYNT to no longer use multiple threads

Along with this fix, we should include:

• better user-friendly CLI options. Most of these were already included here: Big POMDP update #39
• better code quality and test

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Stuff that will not be included with the SAYNT fix:

• new SAYNT loop from CAV SI paper and its related options

This is because most of the changes needed for the new SAYNT loop were never merged into Storm and also the new loop did not provide many significant advantages over base SAYNT.