Small fixes and new features

[ryanlevy]

• Fix identity operator and add some docs (we need a lot more...)
• Beginning of a way to override BCs, should use a similar system like the sites system but we can deal with that later
• Add more of the needed definitions of Digit sites, which allows for product states
• include delta_x and delta_xyz to create a product state bit string configuration

[JoeyT1994]

Good point. Worth commenting this in as its not very straightforward.

[ryanlevy]

(It should move to the doc string eventually too)

[ryanlevy]

• From discussion: swap left,right boundary to tuple and include default of size 1 to exand

[JoeyT1994]

Can you add a compat entry for Graphs.jl to this PR too (v1.8)? Just so we can register the package post this PR

[JoeyT1994]

Looks good to me! Do you want me to merge now and do the implement boundary conditions via a Tuple point above in a separate PR or do it in this PR? Up to you.

[ryanlevy]

Thanks! Lets do that in a different PR and plan out how we want the interface to look, I think it might get a bit messy if we want to allow full heterogeneous conditions

[JoeyT1994]

Happy to merge.