Add SIA numerical modeling section

[jkingslake]

It will be in the ice flow part.

It will need to introduce finite difference.

Then solve the SIA equation with finite difference.

Then do some experiments with the model.

(Note that the numerical methods we will use will be intentionally simple to help with understanding.)

We can also include some extensions: e.g., variable bed topography, debris cover, GIA (maybe).

[jkingslake]

some of this can come straight from the ice-sheet theory seminar: https://github.com/ldeo-glaciology/ice-sheet-seminar-2022

[jkingslake]

#9 made a start on this.

But lots still needs to be done on that chapter

[jkingslake]

#26 added a new page building on the SIA equation.

#19 will add a derivation of the SSA and the Blatter-Pattyn models.