When events for Container

[abeham]

The GasStation example in both SimPy and desmod could be improved by implementing a "WhenAtMost" event for a SimPy Container. The process _monitor_reservoir currently polls every 10s which is inefficient as it generates and processes many events that are actually unnecessary.

I implemented WhenAtMost (level falls below x), WhenAtLeast (level rises above x) and WhenChange (some get or put event occurred) for Containers in SimSharp and updated the sample accordingly.

[jpgrayson]

Thanks for this suggestion--it's a good one. Also thanks for SimSharp it is an impressive body of code!

[jpgrayson]

Desmod's Queue and Pool have been modified to have when_at_least and when_at_most events. The existing when_full, when_not_full, and when_any events are derived from when_at_least and when_at_most--this is a nice improvement to have fewer waiter lists.

The gas station example is also updated to use Pool.when_at_most instead of the timeout-based polling loop.

The one suggestion I did not implement was adding the when_change event. I do not [yet] see a use case for that event.

Thanks again @abeham for these suggestions!