Add StandaloneProblem

[NicolaCourtier]

• Add StandaloneProblem to examples
• Move StandaloneCost in same folder
• Add test for Standalon Problem

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (c3438ab) 94.07% compared to head (3682569) 94.05%.
Report is 3 commits behind head on develop.

❗ Current head 3682569 differs from pull request most recent head 7392620. Consider uploading reports for the commit 7392620 to get more accurate results

Files	Patch %	Lines
examples/standalone/problem.py	92.85%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #153      +/-   ##
===========================================
- Coverage    94.07%   94.05%   -0.03%     
===========================================
  Files           36       37       +1     
  Lines         1351     1379      +28     
===========================================
+ Hits          1271     1297      +26     
- Misses          80       82       +2     

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[BradyPlanden]

LGTM, nice work @NicolaCourtier.

Looking into the future, I think we will want to provide a nicer interface to construct standalone problem and cost classes.

Edit: Can you update the Changelog as well?