added dense versions of row-seq, cell-seq and select-columns for dealing with blanks in spreadsheets

Fix for https://github.com/mjul/docjure/issues/19

Added `dense-row-seq`, `dense-cell-seq`, and `dense-select-columns` for reading sparse spreadsheets without skipping blanks.  Made new functions rather than changing existing ones, as the dense functions may be slower for large spreadsheets, and require extra handling of nil values.

Also fixed docstring for `remove-row!` (and added test) to clarify that rows are blanked, not removed.

Note I think there is still a problem calling `select-name` on a range with blanks, but that should be a separate issue (and separate pull request) when I get the chance.