restructuring the read_scf for stdoutSileSiesta

[zerothi]

This fixes #918 which cause erroneous reads because some details are stored after the scf: key.
Essentially the spin moment is consistently written after.

The current approach in read_scf, is to read data until it hits the same data-key again, at which point it serializes the data and stores it for further processing.

The logic has been streamlined and it should be simpler to follow and edit.

• Closes siesta.stdout reading scf fails with spin-moment #918
• Added tests for new/changed functions?
• Documentation for functionality in docs/
• Changes documented in changes/<pr-num>.<type>.rst

[codecov]

Codecov Report

Attention: Patch coverage is 62.50000% with 39 lines in your changes missing coverage. Please review.

Project coverage is 86.96%. Comparing base (a4945e4) to head (ddea8c7).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/sisl/io/siesta/stdout.py	62.50%	39 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #919      +/-   ##
==========================================
+ Coverage   86.93%   86.96%   +0.02%     
==========================================
  Files         408      408              
  Lines       53836    53857      +21     
==========================================
+ Hits        46805    46838      +33     
+ Misses       7031     7019      -12     

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