Add support for "by" option in mixingmatrix.egodata

[martinamorris]

For example

mixingmatrix("race", by="sex)

Jeanette will look into doing this by writing a "subset.egodata" function. So this issue report is just to note she's getting that started.

[krivit]

1. Please work on the egor branch at this point. egodata class has been deprecated.
2. subset.egodata() already exists in the CRAN version of ergm.ego (and egor also has a subset() method).
3. In the next release, mixingmatrix.egodata() is going to be a method for the mixingmatrix() generic in the network package. It should, ideally, be made compatible; and I am not sure we want a by= argument in the network's generic.
4. Since the return data type won't itself be a matrix but a list or a 3D array, I suggest giving it a different function name, perhaps just mixingmatrices() or mixingarray().

[krivit]

What's the status of this?