better import gds and ports

[joamatab]

• simplify logic for import ports
• add post_process to import_gds

[codecov]

Codecov Report

Attention: Patch coverage is 25.00000% with 3 lines in your changes are missing coverage. Please review.

❗ No coverage uploaded for pull request base (main@0189bb8). Click here to learn what that means.
Report is 4 commits behind head on main.

Files	Patch %	Lines
gdsfactory/read/import_gds.py	33.33%	1 Missing and 1 partial ⚠️
gdsfactory/add_ports.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2680   +/-   ##
=======================================
  Coverage        ?   71.74%           
=======================================
  Files           ?      366           
  Lines           ?    23758           
  Branches        ?     3873           
=======================================
  Hits            ?    17046           
  Misses          ?     5583           
  Partials        ?     1129           

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