Support transformation of columns/headercards?

[kosack]

The problem:

In the VODF work, we realized that there are many parts of the data model where the same information can be used either as a set of columns or a set of HeaderCards, and in many cases both need to be supported. For example observation "headers" are turned into an observation "table" (so headers → columns)

proposed solution:

Since Column and HeaderCard are already quite similar, and have nearly the same attributes, we could consider just merging the two, which would mean separating the way validation works depending on if it is a column or header.

Then a Header class is really just group of headers or columns