Removed interface factory

[rozyczko]

Code:

• Removed InterfaceFactory and associated logic.
  This moves the calculator ownership into the technique specific libraries, and allows a clear separation of responsibilities.

• Removed Parameter's callback keyword and value_no_call_back

• Dropped AnalysisBase(interface=...) and made AnalysisBase arguments keyword-only.

Documentation:

• Added a comprehensive new user guide page, external-calculators.md, explaining how to connect an external calculator to an EasyScience model and Fitter, with diagrams, minimal examples, and best practices for integration and fit control.

Cleanup:

• Removed a number of legacy examples, which demonstrated the outdated pattern.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.99%. Comparing base (bdc119f) to head (adaed61).
⚠️ Report is 2 commits behind head on release-3.0.

Additional details and impacted files

@@               Coverage Diff               @@
##           release-3.0     #333      +/-   ##
===============================================
- Coverage        94.08%   93.99%   -0.10%     
===============================================
  Files               50       48       -2     
  Lines             4094     3946     -148     
===============================================
- Hits              3852     3709     -143     
+ Misses             242      237       -5     

Flag	Coverage Δ
unittests	93.99% <100.00%> (-0.10%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/easyscience/fitting/engine_base.py	100.00% <ø> (ø)
src/easyscience/fitting/fitter.py	96.03% <ø> (ø)
src/easyscience/job/analysis.py	100.00% <100.00%> (ø)
src/easyscience/variable/descriptor_base.py	95.74% <ø> (ø)
src/easyscience/variable/parameter.py	95.22% <ø> (-0.03%)	⬇️

... and 1 file with indirect coverage changes