table.headings(): Contribution and coordinates

[antongrau]

This function is also primarily for publication, because it requires headings to be defined. It is an open question how to properly define the headings. One way might be to simply require a vector with the length of the number of active variables:
"Heading1", "Heading1", "Heading2" etc. This approach is very R like, but not very user friendly.

Another more flexible approach could be to require a list of vectors, where each vector was a heading - but this requires even more R skills. But this would allow for the easy inclusion and exclusion of variables.

This approach requires all variables to only be included in one heading. This might cause a problem for some variables. Another approach could be to require a vector with the length of the number of active modalities. This is a lot more work.

For each heading, the modality is presented with its frequency, contribution and coordinate for 1:3 dimensions.
Each heading has its overall contribution calculated.