Support array-type metadata fields in cubids group

[tsalo]

Closes #281 and closes #331.

A few things to look into

• Fields that are arrays for some subjects and floats for others (e.g., RepetitionTimeExcitation can be either).
• Is the dataframe saved and loaded back into memory at any point? If so I need to find where and use the new utility functions I wrote.
• I think I need to update round_params() too.
• Any other places where metadata fields are compared?

Changes proposed in this pull request

• Allow lists in the param_group_df parameter to cubids.cubids.format_params()
  • Fields with "tolerance" in the config will be fed into the AgglomerativeClustering step, but only after splitting and grouping by the length of the arrays.
  • Fields without "tolerance" will be converted to a string and compared based on that.

[mattcieslak]

This is a very smart approach to the problem. I think it will work.

I don't think any of the dataframes will ever be written and reloaded during the course of a command line call.

[tsalo]

@mattcieslak do you want me to merge?