IndsNetwork additions

[leburgel]

Some changes and additions for IndsNetworks:

• Defined is_directed for AbstractIndsNetwork subtypes so they work with Graphs.SimpleTraits
• Added a uniqueinds method similar to the one for AbstractITensorNetwork.
• Added a merge method for merging AbstractIndsNetworks, which just corresponds to a set union of all site and link indices of the network. Proved quite useful for me, though naming and usefulness in general might be debatable.
• Extended the IndsNetwork constructor to allow specifying the indices for every site/link of the network individually by means of a Dictionary, as well as specifying multiple indices per site/link.
• Extended the siteind system for IndsNetworks to allow passing either an Integer, String or Function sitetype, as well as specifying the sitetype for every site individually. Now also allows passing any Graphs.AbstractGraph subtype, solving Allow passing a Graphs.SimpleGraph to siteinds #7.
• Reorganized tests to use separate files.

[mtfishman]

Thanks @leburgel, this looks good to me. It looks like there are some merge conflicts caused by #13 but once those are fixed I'll merge this.

Gotta get the tests working on this package, it's tough to figure out what's going on when dealing with unregistered packages...

[leburgel]

Conflicts are resolved and tests pass locally, no idea how to fix the actual CI though.

[mtfishman]

In #13 I deleted the file examples/peps/utils.jl since I moved it all into the library, but it looks like you added it back during your merge.

[mtfishman]

(I think I'll just have to bite the bullet and register all of the dependent packages to get CI working better.)

[leburgel]

In #13 I deleted the file examples/peps/utils.jl since I moved it all into the library, but it looks like you added it back during your merge.

My bad, removed it again.

[mtfishman]

Thanks again for the contribution!