Introduce table's column's width property

[Intelligent2013]

Introduce width property for each column in table for better display in PDF and allow transforming Metanorma XML into NISO STS.
NISO STS format has such attribute @width for 'col' elements:

<table width="638">
<col width="14%"/>
<col width="15.09%"/>
<col width="17.92%"/>
<col width="26.41%"/>
<col width="26.58%"/>
<thead>
<tr>...

[opoudjis]

Tightening the requirement on the cols attribute: each column needs to be given an explicit width number, in order for there to be a valid colgroup element, and there needs to be more than one column. [cols="2"] will not be recognised as having equal width, but [cols="1,1"] will.

[opoudjis]

The Metanorma XML is:

<table>
<colgroup>
<col width="...%"/>
<col width="...%"/>
<col width="...%"/>
</colgroup>
</table>

The colgroup element is optional.