Add method to subset Baseline objects

[Jason Vander Heiden]

Need a function to subset a Baseline object by annotations in the db slot, which would subset both the db and pdf matrices. Would be nice if it acted just like the subset function.

[Jason Vander Heiden]

We've had two requests for a merge method as well. Though, I suspect that won't be necessary if we can reduce the memory usage.

[Julian Zhou]

I have mixed feelings about these requests. I feel it might be quite difficult to come up with a catch-all kind of generalizable function that does subsetting/merging BASELINe objects (based on varying specifications of each user -- that's the hard part). Both tasks are doable using createBaseline() and/or editBaseline(), and are not hard. The only caveat is that one would have to know BASELINe reasonably well in order to know how to do it.

[Jason Vander Heiden]

Well, this issue was opened almost 3 years ago and I haven't needed a subset function in all that time, so... we probably don't need it. I don't think I've ever used editBaseline either.

The two recent requests for a merge function were for different reasons too. One because of the memory problems, and the other to combine results from different RegionDefinitions. The former is (probably) fixable through other methods and the latter can't be done regardless.

[Jason Vander Heiden]

Let's say "no" for now. Reopen if it ever comes up again.