Distinguish 'D2S Property' and 'D2S Header' cells

[RinkeHoekstra]

TabLinker currently does not properly distinguish between property and header cells. Properties are meant to relate data values to row headers (i.e. across the table), while header cells should only be related to values in their own column. This means that the dimcol dictionary should be split into two dictionaries: one for properties and one for headers.