move slow import inside function call

[atravitz]

For cli commands, slow imports should be moved inside functions (as recommended here).

By moving the plan_alchemical_network_output import from the top of the file to inside the functions for rbfe and rhfe network planning, we avoid importing openfe during the command-gathering process.

The --help and --version calls now take 0.15 seconds instead of ~4 seconds.

Ideally, we will also speed up import openfe, which was the root cause of this slow behavior, in another PR.

Checklist

• Added a news entry

Developers certificate of origin

• I certify that this contribution is covered by the MIT License here and the Developer Certificate of Origin at https://developercertificate.org/.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.84%. Comparing base (29d8cd9) to head (0186b3c).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #955      +/-   ##
==========================================
- Coverage   94.59%   92.84%   -1.76%     
==========================================
  Files         134      134              
  Lines        9935     9935              
==========================================
- Hits         9398     9224     -174     
- Misses        537      711     +174     

Flag	Coverage Δ
fast-tests	92.84% <100.00%> (?)
slow-tests	?

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[IAlibay]

lgtm thanks!