add support for reading in MUSIC2 initial conditions

[BenWibking]

Describe the proposal
Add the ability to read in particle ICs computed using MUSIC2 (https://github.com/cosmo-sims/MUSIC2).

We need to deposit the gas particles onto AMR grids (with refinement).

Describe alternatives you've considered
N/A

Additional context
This will let us do cosmological zoom-in simulations, e.g., for PopIII stars.

cc @psharda

[BenWibking]

We should (in principle) be able to read in the Nyx outputs: https://github.com/cosmo-sims/MUSIC2/blob/master/src/plugins/nyx_plugin/output_nyx.cpp

[BenWibking]

FYI This is very cool: https://cosmicweb.eu/