Add options for mixed PBC/OBC and direction-colored edges to Grid

Created by: femtobit

This PR extends Grid to support mixed boundary conditions by passing a list of booleans to pbc=. It also adds an option to color the edges by direction in order to make it easier to define anisotropic models.

Grid(length=[8, 8], pbc=[True, False])  # cylinder
Grid(length=[8, 8], color_edges=True)   # differently colored x/y edges

Under the hood, this PR also changes how the color is stored in NetworkX graphs: Previously, it was stored as the key of an edge, which is a networkx.MultiGraph-feature meant to distinguish multiple edges between the same node. Now, I use an extra attribute "color" on each edge. This makes the implementation easier (attributes can be added and changed more easily than edge keys) and could allow us to use normal graphs where a multigraph is not needed in the future.