ENH: make rocketpy classes json serializable

[Gui-FernandesBR]

Is your feature request related to a problem? Please describe.

Exporting rocketpy classes to a json file is something that the user currently cannot do so easily.
The get_instance_attributes (see below) function already made an important advance to export all the attributes of an instance to a dictionary, but that doesn't mean we can always save these dictionaries to json files.

Exporting objects as json files might be the easiest way of saving a rocketpy simulation without having to plot tons of data and plots.

RocketPy/rocketpy/utilities.py

Line 642 in 14dd0f6

def get_instance_attributes(instance):

Describe the solution you'd like

• Add to_json, to_serializable or __json__ methods to the important classes of RocketPy, allowing the
• Alternatively, the jsonpickle package can be used to perform a quick encode/decode with any of the rocketpy classes, as already done in the EnvironmentAnalysis class.
  

  RocketPy/rocketpy/environment/environment_analysis.py

  Line 2899 in 14dd0f6

  def save(self, filename="env_analysis_dict"):

Additional context

As far as I know there's no convention on Python regarding the name of the method