feature_name not actually converted to feature_id across all package

[barbarazpc]

molecules(me, assayName = "detected", flatten = TRUE) shows that feature_name still ends up being the column name. But it should be feature_id. This is apparent when using the ggplot_me functions:
-byFillargument still needs feature_names. Should be changed to feature_id

Also:

• geom_polygon_me should accept assay names different than "cell" as the default, or at least warn user.