Set operations and `rand` implementation

[haberdashPI]

These changes set up some functionality I'm using to generate null annotation labels: e.g. to sample regions of data that have no annotation.

This is still in draft form, working on defining some tests...

• add support for Base functions union, intersect and setdiff, etc...
• add a type, TimeSpanUnion to establish and maintain invariants on the output of set operations; these are read-only arrays of TimeSpans
• add Base.in method (already exists)
• add support for Base.rand over TimeSpan and TimeSpanUnion types.
• adds an extend method to adjust the endpoint of a TimeSpan.
• add invariant preserving operations translateall(!) and shrinkall(!) over TimeSpanUnion objects.

[haberdashPI]

One comment: I did look into using an extension of IntervalSets called IntervalUnionArithmetic, but IntervalSets assumes the values are <:Real in its implementation 😞 .

[haberdashPI]

Looks like I pulled the trigger on requesting reviews a little early; there are still a few minor bugs to work out (though I guess that doesn't need to stop someone from looking over what I do have working).

[haberdashPI]

Should be ready for reviews now!

[haberdashPI]

Should be ready for any further reviews, if needed.

[haberdashPI]

Awaiting on discussion #10 to resolve before I make any changes here (since this may move to a PR for Intervals.jl).

[haberdashPI]

Based on discussion in #10, I'm planning to make a PR to Intervals.jl with this functionality.