:result-datatype is unused in invert-categorical-map

[behrica]

When transforming the categorical columns of a datset to numeric via ds/categorical->number the attached categorical map contains a field "result-datatype". (and it can be specified manaully as well)

This is unused when we transform "back" using the function 'reverse-map-categorical-xforms' which uses
'invert-categorical-map'

Not sure, if this is a bug or only misleading to me.

It is true that the "pair" of:

``` `fit-categorical-map
transform-categorical-map``

uses it
But these seem to be two different path ways.

[behrica]

I should maybe add, that I use exclusively the pair of functions:

tech.v3.dataset/categorical->number
tech.v3.dataset.categorical/reverse-map-categorical-xforms

as both take a dataset as input, which matches "my use cases" of categorical columns for ML.

(I was alreday wondering, why there is not function tech.v3.dataset/reverse-map-categorical-xforms)