Probability of failure risk heat map

Write a vignette as a proof of concept that will be reuse in #14 for displaying the plots for a given objective at a given station (Let's say Paris, 950 m3/s high flows).

Subtasks:

• Write a function to extract and sort reservoir volumes from 'PaChrono.txt'
• Write a function to calculate the probability of risk for a given calendar day and a given volume in one reservoir
• Write a function for plotting the heat maps of all reservoirs concerned by the current objective for a given discretisation volume step (corresponding to the vertical pixel size in the heat map)