Option to anchor metadata to specific columns

[jeremymiller]

• This allows for situations like the AD one, where everything can be mapped to SEA-AD but NOT to each other
• Part of this is having these anchor metadata only show up in left side of multi-taxonomy comparison panel
• Part of this also is having river plots from non-anchor columns only sit next to anchor columns and limiting pairwise comparisons to anchor-nonanchor and anchor-anchor, but NOT nonanchor-nonanchor.