IMP: Update --p-reference-levels input type within ANCOM-BC as a Collection

[lizgehret]

Currently the --p-reference-levels parameter within ANCOM-BC takes strings as input, and splits the column::value pairs on a double colon. While uncommon, this becomes problematic for any users who have double colons in their metadata.

The best solution for this would be to modify this parameter to accept a Collection type, which would then turn this input into a dict of key:value pairs, and there would be no restrictions on any characters included within a user's metadata.