add worksheet to cell_addr?

[jennybc]

In planning session with @richfitz, we identified a need for a new S3 class, maybe called cell, that holds row, column, and, optionally, worksheet. We said "possibly vectorized". This is almost exactly what the new class cell_addr already is. It only lacks the worksheet info. Should I just add that? Would it be vectorized as row and col are? I think not? I think worksheet is probably a single value that applies to all cells in the object.