Add multi-subchannel support.

[CFlaniganMide]

It's desirable for users to be able to access multiple subchannels of a source channel in a single coherent structure. For example, accessing accelerometer data from an ADC which also has a microphone on it.

The EventArray class effectively already supports this; all that's really required is a source (MetaChannel maybe?) which only lists the desired subchannels and a new PolyPoly object which only have the relevant polynomials.

[StokesMIDE]

As described, this sounds more like a filter to exclude subchannels, which is fairly straightforward. It could be an argument in Channel.getSession(): a list of channel IDs, and/or a measurement type (from the work being done for the endaq libraries).

If the idea is to also allow combining subchannels from multiple channels, it gets difficult. The samples don't line up between channels, and I'm not sure it's a good idea to provide interpolated data without explicitly being requested, or at least without somehow identifying it.