Put ashm_multi_rainbow_plot where it works

[rdmorin]

Relocated to this package to make this function still useable

Pull Request Checklists

Important: When opening a pull request, keep only the applicable checklist and delete all other sections.

Checklist for all PRs

Required

• I ensured not to prepend the functions existing in both GAMBLR.data and GAMBLR.results with <package>::function() syntax.
• I ensured not to add GAMBLR.data or GAMBLR.results to the @import section of the documentation