Create an ScColumnsRange class that returns a pair of (start,end)
iterators to go through the list of currently allocated columns.
This is a fairly thing wrapper around the underlying std::vector,
so it should be fairly efficient (two pointers, and pointer increment
for iteration).

If this style of iteration is acceptable, I'll go through the rest
of the code that does:

    for (SCCOL nCol=0; nCol<MAXCOLCOUNT; nCol++)

type stuff, and change it to use ScColumnsRange.

Change-Id: I81501c69b7f5566c6204dde0d87a6fe0deb9743c
Reviewed-on: https://gerrit.libreoffice.org/41413Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>