Allow arbitrary subset of donors for each SC

[Brian Quistorff]

There are two common scenarios where one would like to not use all donors for a SC.

1. SUTVA violation: There might be local effects that cause "contamination" between units (e.g. if units are physical and border each other).
2. Interpolation bias: You might want to disallow donors that have very different X since they might be included in the SC if there are other offsetting units.

I think the best way to enable this is to allow user to pass-in a valid-donor-for matrix with binary values.