binocular data

[ihrke]

Create a BinocularPupilData object that can simultaneously represent data from both the left and the right eye.
Internally, this class can keep two PupilData-objects and pass on all the operations to both datasets.
Another option is to inherit from PupilData, keep two extra arrays for left and right eye and then switch out the sy field before running each operation twice.
Both options require re-implementation of all functions, though. Is there a better way?

Also, allow to reduce to a standard PupilData object by averaging or regression (see #7).