issue with plotting covariates and faceting plots uniformly across methods in panpipes_integration.py

[deevdevil88]

hiya!
i noticed this while looking at plots for the different covariates across the multiple batch correction methods, when running integration workflow from panpipes. The same colours are not use to depict the same legend categories (in my case i noticed it for VDJ receptor subtypes) across all the methods, and when facet plots are created, the order of the headings is different for each method. The latter isnt a problem as such, but does make it difficult to compare across methods in a facet plot. Not sure if the first plottign issue happpens for all covariates or not, or only certain types. Thought i would flag the issue.

best,
Devika