Use abs in convergence check

[bkraske]

Uses absolute values in the convergence check to accommodate problem formulations which converge to negative values using value iteration.

[Zinoex]

Hi Ben, thanks for the PR! Could you clarify when this is needed? The value iteration over the properties defined in this package is monotonously increasing, so the difference should always be positive. Maybe when using the InfiniteTimeReward property?

I would like to see a test case to go with this change. If it is indeed InfiniteTimeReward, you could take a look to see if the following code fails due to the lack of abs and if so, we could comment this in and it would be the test case.

[Zinoex]

I see the problem now - in particular, for FiniteTimeReward and InfiniteTimeReward whenever any of the immediate rewards are negative (possibly an issue for FiniteTimeSafety and InfiniteTimeSafety too). I have added appropriate tests and run the JuliaFormatter.

[bkraske]

Apologies for the lack of detail in my initial comment and delay in response. I believe that covers the case I had in mind.
Thank you!