Low-thrust porkchop example, radiation pressure update, some other fixes

[alopezrivera]

This PR contains the following updates:

Astro:

• Shape-based low-thrust porkchop example including trajectory visualizations to verify the porkchops, as well as some discussion on further steps to design low-thrust trajectories.
• Updated all radiation pressure models used in the examples to the new interface

Infra:
All changes implemented in the a new script -create_scripts.py- which substitutes clean_py_notebooks.py and convert_py_to_ipynb

• Improved the generation of Python scripts from Jupyter notebooks
  • Script creation (using Jupyter nbconvert) and cleanup automatically done from create_scripts.py
  • Removed a few instances of syntax errors caused by the cleanup of the generated Python scripts (the process which used to live in clean_py_notebooks.py)
• Syntax verification of examples: added step in create_scripts.py syntax check warning of generated Python scripts
• Functional verification of examples: added option in in create_scripts.py to check all Python scripts (those with no syntax errors)

[alopezrivera]

DO NOT MERGE YET: awaiting verification of pygmo examples