Support isStartIncluded/isStopIncluded on intervals

[shunter]

Currently, values specified via interval always implicitly include the start and stop times, and subsequent intervals always replace previous intervals.

That is, if you have value 1 from t1 to t2, and value 2 from t2 to t3, there is no way to indicate that the value at t2 should be 1 rather than 2.

Cesium has isStartIncluded and isStopIncluded flags which already handle this on the client. Not sure what the syntax should be. Perhaps ISO interval syntax has a way to do this, though I doubt it.