Set up compute layer for density

[mattwthompson]

This is part of a larger effort, but the scope here is trying to define precisely what we want the user to be able to control when. This is analogous to what happens under the hood in Evaluator when we do

from openff.evaluator.properties import Density

density_schema = Density.default_simulation_schema(n_molecules=256)

I don't currently care much if this lives in the same class as storing the ThermoML data; to me, it makes more sense to separate out targets (parsing data, defining thermodynamic state, etc.) from how to compute/estimate them (system- and simulation-specific details)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.57%. Comparing base (8675326) to head (9538f6f).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #73   +/-   ##
=======================================
  Coverage   75.57%   75.57%           
=======================================
  Files          10       10           
  Lines        1265     1265           
=======================================
  Hits          956      956           
  Misses        309      309           

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