Tabular module should support merged cells in HTML and XLS

[blcham]

• Investigate how it is possible to merge columns in HTML (maybe span column property?)

A/C:

• it is possible to query output of the module to find out which cells were merged (from HTML and XLS formats)

[blcham]

I just started to think about it :

• it does not make sense to merge whole rows or whole columns (as it just became single row/column), thus we are not really merging columns or rows but cells
• we can consider one cell as master cell in merged group of cell -- i.e. the most top-left cell we can call master and other can be said that are just "duplicates"
• most of the cases for having merged cells is to provide hierarchies in header

My first idea is to represent cells using csvw extension namespace kbss-csvw:

ex:some-master-cell a kbss-csvw:Cell;
  ex:row ex:row1;
  ex:column ex:coll1;
  kbss-csvw:same-value-as ex:cell-2 ;
  kbss-csvw:same-value-as ex:cell-3 ;
.

ex:cell-2  a kbss-csvw:Cell;
   ex:row ex:row1;
   ex:column ex:coll2;
.

ex:cell-3  a kbss-csvw:Cell;
   ex:row ex:row1;
   ex:column ex:coll3;
.

kbss-csvw:same-value-as should be transitive and symetric relation but we for us it is sufficient to assert it from "master" cell to "slave" cells.