Fix mutation behavior of position

[b-kloss]

This PR makes position(pos, PH) not mutate PH::AbstractProjTTN. Non-mutating versions of make_environment are also implemented for the concrete subtypes. Additionally, copy is implemented for the concrete types using copy_values_keys on PH.enviroments to provide copy safety (not entirely necessary if only in-place version is kept) and the original implementation renamed to unsafe_copy.
A test example verifying copy safety is provided (the original implementation was not copy-safe due to
andyferris/Dictionaries.jl#98) .

ToDo:

• bring copy_keys_values into namespace
• test whether it makes sense to keep out-of-place version for performance?
• add test for out-place version if kept
• also modify ProjTTNSum ?

[b-kloss]

Any idea why the test was canceled?

[mtfishman]

Any idea why the test was canceled?

No idea, could just be an issue on Github's side. We can try restarting them.

[mtfishman]

Seems like we could change all uses of ::TTN to ::AbstractTTN to make it more general.

[mtfishman]

I think overall the new code is definitely an improvement, but needs a cleanup and style improvements based on my comments.

[codecov-commenter]

Codecov Report

Attention: 11 lines in your changes are missing coverage. Please review.

Comparison is base (56fcb93) 72.90% compared to head (887d913) 72.87%.
Report is 3 commits behind head on main.

Files	Patch %	Lines
src/treetensornetworks/projttns/abstractprojttn.jl	63.33%	11 Missing ⚠️

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #131      +/-   ##
==========================================
- Coverage   72.90%   72.87%   -0.03%     
==========================================
  Files          71       68       -3     
  Lines        4097     3982     -115     
==========================================
- Hits         2987     2902      -85     
+ Misses       1110     1080      -30     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[mtfishman]

Looks good, thanks @b-kloss. This kind of cleanup/refactor will help when generalizing to loopy graphs with BP (@JoeyT1994).