Expose row and cell complex types

[BassP97]

This MR, once merged, will allow for more advanced table row/cell manipulation and formatting, including, eg, row height:

    rowCt := row.GetCt()
    rowCt.Property.Height = ctypes.NewTableRowHeight(500, stypes.HeightRuleExact)

And cell width:

    cell1Ct := cell1.GetCt()
    cell2Ct := cell2.GetCt()
    cell3Ct := cell3.GetCt()

    cell1Ct.Property.Width = ctypes.NewTableWidth(2000, stypes.TableWidthDxa)
    cell2Ct.Property.Width = ctypes.NewTableWidth(3000, stypes.TableWidthP
    cell3Ct.Property.Width = ctypes.NewTableWidth(0, stypes.TableWidthAuto)

It might be cleaner to expose cell-row level apis for different props... but this is simple and straightforward :)

[BassP97]

Bump! I'd love to merge this :)