Lazy loading of the viz module

[pfebrer]

• Closes sisl.viz lazily loaded #690
• Added tests for new/changed functions?
• Ran isort . and black . [24.2.0] at top-level
• Documentation for functionality in docs/
• Changes documented in CHANGELOG.md

As proposed in #690, now the sisl module is lazily loaded, so no need for explicitly importing sisl.viz or the existence of the SISL_VIZ_AUTOLOAD variable anymore.

I updated the documentation accordingly.

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 75.68%. Comparing base (02f931e) to head (c31b395).

Files	Patch %	Lines
src/sisl/_lazy_viz.py	76.47%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #692   +/-   ##
=======================================
  Coverage   75.68%   75.68%           
=======================================
  Files         397      398    +1     
  Lines       50393    50418   +25     
=======================================
+ Hits        38139    38161   +22     
- Misses      12254    12257    +3     

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[zerothi]

Thanks!