[gconstruct] Multi-column features should save indpendent transformation for each column

Currently GConstruct saves a single transformation in the output gcon config even when the input has multiple columns.

We should be saving one transformation per column and apply them individually (e.g. min-max needs per-column min and max value to be re-applied, as GSProcessing does)

Start by locating gconstruct's handling of transformations and the output gcon configuration, then compare the per-column min-max behavior described for GSProcessing. Done means each input column has its own saved transformation parameters and those parameters are applied independently when reused.

Written by the indexing model from the issue text.