[ITensors] Remove Observers dependency

[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.

(Another attempt, making incremental changes to see what is breaking the tests. All tests pass locally but not in the CI.)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.59%. Comparing base (e8197ce) to head (c7025f8).

❗ Current head c7025f8 differs from pull request most recent head 519e3e5. Consider uploading reports for the commit 519e3e5 to get more accurate results

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Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1379   +/-   ##
=======================================
  Coverage   79.59%   79.59%           
=======================================
  Files         114      114           
  Lines        9032     9032           
=======================================
  Hits         7189     7189           
  Misses       1843     1843           

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[mtfishman]

@emstoudenmire we could just merge this and then work on the extension in a followup, we may as well at least remove the dependency.

[emstoudenmire]

That sounds good. In the exten@ PR my plan is to make small changes until I get that Pkg bug again, to narrow down why. It's probably to do with Project.toml (maybe even the test/ did ones?) and maybe weak deps. We will see.