Implement heterogeneous number of trials in binomial-reference ERGM.

[krivit]

Some use-cases:

• n(i,j) arbitrary. Probably have to pass an adjacency matrix (or close) of values.
• n(i,j) with block structure. Requires a Run-Length-Encoded Valued Dyad Matrix that stores runs of nonzero values?
• n(i,j) := min(a[i],a[j]), which arises in collapsed bipartite networks. Probably implementable as a special case.