Density-likes should not be wrangler-global

[inducer]

Specifically, this code here:

https://github.com/inducer/boxtree/blob/d48fcdfb1f6533a7cd854ea861a98e1c69055e90/boxtree/pyfmmlib_integration.py#L379-L394

should not exist.

cc @gaohao95

[inducer]

It turns out that source arguments are wrangler-global in sumpy as well:

https://github.com/inducer/sumpy/blob/e1048b0a4c0c8512d0765e4eef09963dcc77ab0f/sumpy/fmm.py#L314

So both need to change.

[inducer]

So the way that this should go IMO is that we need to change the drive_fmm interface to take in a dictionary (containing densities, and, if needed, dipole vectors and other per-source arguments).