MNT: move piecewise functions to separate file

[yogirajgutte]

closes #667

Pull request type

• Code changes (bugfix, features)
• Code maintenance (refactoring, formatting, tests)
• ReadMe, Docs and GitHub updates
• Other (please describe):

Checklist

• Tests for the changes have been added (if needed)
• Docs have been reviewed and added / updated
• Lint (black rocketpy/ tests/) has passed locally
• All tests (pytest tests -m slow --runslow) have passed locally
• CHANGELOG.md has been updated (if relevant)

Current behavior

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New behavior

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Breaking change

• Yes
• No

Additional information

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.47%. Comparing base (83aa20e) to head (a997774).
Report is 6 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #746      +/-   ##
===========================================
+ Coverage    76.42%   76.47%   +0.05%     
===========================================
  Files           95       96       +1     
  Lines        11090    11094       +4     
===========================================
+ Hits          8475     8484       +9     
+ Misses        2615     2610       -5     

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[Lucas-Prates]

@Gui-FernandesBR and @phmbressan, besides applying the code formaters and creating the tests, I simplified and optimized just a bit the PiecewiseFunction class. If you can, please take a very quick glance just to make sure it LGTY.

[phmbressan]

Before merging, the CHANGELOG should be updated.

[Gui-FernandesBR]

LGTM