[ITensorMPS] Observers package extension

[emstoudenmire]

This PR introduces a simple package extension that defines ITensorMPS.update_observer! when the Observers package is loaded, and a simple test for it. It has been locally tested against the ITensorTDVP package also.

[emstoudenmire]

Also I could add the PackageExtensionCompat dependency, but since it is being added in #1373 I hesitated to do that.

[mtfishman]

Thanks. We should use PackageExtensionCompat instead of the if !isdefined(Base, :get_extension) ... pattern. I'll be merging #1373 soon but the order of operations doesn't matter there.

However, before making any more changes to ITensorsMPS I think we should get downstream tests of ITensorTDVP working (#1370 (comment)).

[emstoudenmire]

Agreed about getting the tests going before merging this.

Also if you have any guesses why the ITensors and NDTensors tests are failing here I'm a bit at a loss as to why.

[mtfishman]

Not sure what's going on there, the configuration looks good to me.

[mtfishman]

Should this be closed?

[emstoudenmire]

Yes I just closed it. I'll use it as a reference for another PR or maybe two smaller ones.