Demo for custom classes

[RemDelaporteMathurin]

This fixes #59

[jhdark]

Just a few minor comments fom me:

• It might be worth mentioning at the beginning that what we would like is the advection flux, showing the equation, and noting that this type of export is not currently implemented.
• We can then note that what we want can be inherited from the SurfaceQuantity class and maybe highlight how inheritance works. Could maybe also provide a link to a tutorial on class structures? (https://www.w3schools.com/python/python_inheritance.asp)
• can remove the comment "reinitialisation is needed". This is the first time as no T component in the velocity field
• is it worth printing the units of the advection flux too?
• I understand the plot of the Recomb to Surface flux is just to show that it matches, but I wonder if its worth adding axis labels here, maybe not necessary
• do we need to add scipy to the dependencies or is that done via the festim installation?

[RemDelaporteMathurin]

@jhdark thanks, yes I didn't put too many efforts in documenting this demo. I'll add these details

[RemDelaporteMathurin]

@jhdark I added some more description. Let me know what you think