Add 3di as a data type

[roblanf]

The structural phylo people (https://biosig.lab.uq.edu.au/strphy26/program/) would like it if we included some things into IQ-TREE / ModelFinder. MF2 is a great opportunity to do this.

A couple of useful things here:

1. Include 3Di as a data type
2. Include 3Di models in the built-in models (there are a few of these that are published)
3. Make it easy to specify mixed data types in alignments (e.g. AA partitions, 3Di partitions) have ModelFinder2 elegantly deal with merging in these mixed data partitions.

Let's use this thread to discuss these and other potential inclusions for MF2.

pinging @jordandouglas and @nmatzke (and can someone add Caroline - I can't find her GitHub).

[roblanf]

Ah, and once we have this stuff done, we should add a detailed 3di tutorial, something to replicate a published analysis. Here's a good template:

https://biosig.lab.uq.edu.au/strphy26/static/StrphyWorkshop2.pdf

StrphyWorkshop2.pdf

[roblanf]

adding @carolinePL