MNT: Move the Piecewise function to a separate module

[Gui-FernandesBR]

Reasons for solving this issue:

• The function.py file is already quite big (more than 3.5k code lines).
• We should define each class in a separate file, this makes the lib more readable.
• There is absolutely no need to define the Piecewise Function class in the same file as the Functionclass

Steps for accomplishing a solution to this issue:

• Create a file named rocketpy\mathutils\piecewise_function.py
• Move the class PiecewiseFunction(Function): definition to the new file
• Updates the mathutils/__init__.py file to ensure the imports are working
• Run the tests (make pytest) and verify everything is working properly
• Beware of potential breaking changes. What is the behavior of the following command before and after the changes?

from rocketpy.mathutils.function import PiecewiseFunction