Improved porkchop example, fixed thumbnails

[alopezrivera]

This pull request:

• Includes data management functionality (saving data using the pickle Python core library) in the earth_mars_transfer_window.ipynb notebook demonstrating porkchop plots
• Adds a thumbnail for the earth_mars_transfer_window.ipynb notebook
• Attempts to fix the thumbnail of the mga_trajectories.ipynb notebook

The latter two look like this on the Tudat site at present:

[DominicDirkx]

Thanks! Could you also add the modified .py file for the earth_mars_transfer_window.ipynb?

[alopezrivera]

Done!

[DominicDirkx]

Nice, thanks!!