Add example of limiting particle timestep by local cfl in schism tuto… - #3016

"Altitude": reworded. The text now describes the cell width as the shortest distance from a corner of the triangle to the opposite edge (twice the area divided by the longest edge), and notes that this is one of several valid measures.

Variable names: p, e, h are now corners (points), edges, edge_lengths, cell_area and cell_width, both in that cell and in the kernel and release cells.

Moving the cell: the cell-width code now lives in the "Limit the timestep by the local CFL number" section, together with the kernel. It opens by introducing the CFL number, $C = |u|,\Delta t/\Delta x$. To make the move possible, cell_width is no longer part of the velocity UxDataset. Instead it's built as its own small FieldSet and added with fieldset = fieldset + .... this doesn't seem to impact runtime for this example

$C$ vs CFL_target: now stated explicitly: "with $C$ = CFL_target and $\Delta x$ = cell_width". There's also a sentence on how fieldset.add_context makes CFL_target available in the kernel.

Link to tutorial_dt_integrators: added. I linked to "A priori estimation of dt", which makes the same $U,\Delta t < \Delta x$ argument for a structured grid.