ENH: Clean Plots and Prints Subpackages init files

[MateusStano]

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 --runslow) have passed locally

Current behavior

Because of some circular imports errors, the __init__.py file of the plots subpackage was changed to be empty. This is not an issue because the plots classes are only meant to be imported in other files of the library. However, the CompareFlights class is in this subpackage, and it is meant to be easily imported.

New behavior

The prints subpackage __init.py__ file is also empty and the imports for the Compare classes have been added

Breaking change

• Yes
• No

Additional information

Just to make things clear. Currently, in master we can do:

from rocketpy.plots import _EnvironmentPlots

or

from rocketpy.plots.environment_plots import _EnvironmentPlots

and for compare we had to do

from rocketpy.plots.compare import CompareFlights

With the changes of this PR, we can do only

from rocketpy.plots.environment_plots import _EnvironmentPlots

and for compare we now can do

from rocketpy import CompareFlights

or

from rocketpy.plots import CompareFlights

[Gui-FernandesBR]

Well done!
The plots and prints modules are more consistent now.