Adding GP Tutorial

[Gaurav17Joshi]

This PR is for adding a tutorial for the Gaussian Process tool in the stingray.modeling file. The contents of the tutorial include

• Use of GP's in Astronomy
• Making a sample lightcurve for modeling
• Finding Evidence of Sample Lightcurve for a Red Noise Model using the stingray.modeling.gpmodeling functions
• Finding Evidence of Sample Lightcurve for QPO Model
• Using the Plotting features of GPResult class to visualise the Nested Sampling run which calculated the Evidence

Any points which improve the clarity of the interface explanation would be helpful