When creating a MTable from HTML we should compute each column width.

[ylussaud]

This would allow to add a CTTblGrid to the created XWPFTable. This is mandatory for pdf conversion (see #357) and would probably reduce rendering differences when opening generated documents with LibreOffice. This is probably not an easy task since it will require to compute the size of the content of each cell and probably adapt it to the size of the containing table or page and apply the width policy.

NOTE: a width attribute has already been added to MCell see #472.