crosstab: add categorical vector input support

[beingamanforever]

This PR adds support for categorical array inputs to crosstab, addressing issue #340.

Categorical vectors are detected via a guarded iscategorical check and normalized
using the existing grp2idx infrastructure. All original behavior is preserved.

New BISTs are added for categorical-only inputs and are skipped automatically
when categorical support is unavailable.

String and table support are intentionally not included and can be handled
in follow-up PRs after maintainer discussion.

[pr0m1th3as]

The check for categorical support is useless. statistics already depends on datatypes, which makes categorical arrays available for the statistics package.

Do not remove or modify existing BISTs, unless they are testing against wrong output (which is not the case).

Since crosstab uses grp2idx internally, adding support for categorical arrays needs a much less involved change. It is always preferable to keep the code as simple as possible.