Graphene now inherits from Molecule

[chrisjonesBSU]

Given some discussion with @marjanAlbouye about how to best design a surface wetting module, one of the first to-dos was making it so that the surfaces in flowermd.library.surfaces inherit from flowermd.base.Molecule. This PR makes this change.

Here is an example of how we can create a graphene surface and apply a forcefield:

from flowermd.library import Graphene
from flowermd.library import OPLS_AA
from gmso.external import to_hoomd_snapshot, to_hoomd_forcefield

graphene = Graphene(
    x_repeat=4,
    y_repeat=4,
    n_layers=2,
    force_field=OPLS_AA()
)

graphene_snap = to_hoomd_snapshot(graphene.gmso_molecule)[0]
graph_ff = to_hoomd_forcefield(graphene.gmso_molecule, r_cut=2.5)[0]

This was the first step needed in order for the design choices we discussed in a class that combines a surface with a droplet GSD file.

[marjanalbooyeh]

LGTM. Thanks Chris!