start at declaring traits in the slim code

[petrelharp]

This is a start at things - and maybe not a good start! Just trying things out.

edit: This PR has code to the following:

1. run tests using (development) multitrait SLiM (here)
2. initializes traits
3. sets up mutation types and effect size distributions for traits
4. as well as the mutation callback for multivariate traits (currently just mvn)
5. sets individual phenotypes and implements transforms

TODOs: should be done or have an issue opened to track

• provide consistency check for traits and trait model
• clarify how "fitness" is going to work? currently there's a trait called "fitness" which has direct effects on fitness; is this what we want? this shoudl be documented somewhere? Right now the code has the effects of new mutations 0 here but then later sets the effect size distrn for trait here; is this doing the right thing?
• make the indentation in the SLiM script consistent
• remove this monkey patch maybe?
• write docstring for traits model argument to simulate
• write is_neutral

[petrelharp]

Right now on my machine I get

>               raise ValueError(
                    "Invalid json+struct payload: declared lengths exceed buffer size"
                )
E               ValueError: Invalid json+struct payload: declared lengths exceed buffer size

../../../micromamba/envs/ts/lib/python3.13/site-packages/tskit/metadata.py:318: ValueError