try to preserve alignment in cells

It appears that columns always have a `c` directive, which requires manual editing after the table has been translated — which has to then be remembered and done afresh each time the table is re-generated. It would be awfully helpful if each column's alignment


could be recorded in the generated table (and cell's alignment adjusted when it diverges from that of the column). Since there are only three forms of alignment (for now) in Sheets, and they each have simple LaTeX counterparts, hopefully this isn't too onerous.