Add a carbon fiber initialization scheme

We need a way to initialize systems and run simulations that will help us model carbon fiber wetting. I have an mbuild function that will generate orthogonal sheets of graphene layers that we can use in the simulation volume. We'd have to make sure to adjust box sizes in a way that account for the desired density of the polymer melt (i.e. expand the box by the thickness of the fiber sheets).

Review the existing system-initialization flow and the mbuild function that generates orthogonal graphene sheets. Determine how the simulation volume and polymer-melt density are currently configured. Done means a carbon-fiber initialization scheme exists, uses the sheets in the simulation volume, and accounts for their thickness when sizing the box.

Written by the indexing model from the issue text.