Standardize use of counting terms in 2.1 encodings

[westonpace]

We used the terms "row, element, item, visible item, and value" all over the place (e.g. num_rows or num_items) and in many places they mean different things. This has led to counting errors and general confusion. A standardized set of terms is proposed in #3344 and, once that merges, we should apply these terms consistently in the rest of the code base.