Taal: Support for restarting simulations

[sloede]

This issue is to track the discussion on how to implement restarting in Taal. From Slack on Oct 20th, 2020:

Michael
As a general question (since I forgot): Does Taal support writing restart files and restarting from them yet?

Hendrik Ranocha
I haven't implemented that part yet. There is an open check box in the main Taal PR for this. I think we just need another callback or enhance the savin callback to write restart files. Loading from restart files should probably be done in the elixir by using some convenient methods.

Michael
OK, thanks for the quick reply. Maybe we can somehow dispatch on the initial condition not being a function but a filename (or similar), which then would load data from there and reset dt and time in the solver

Hendrik Ranocha
Sounds also good to me. You can give it a shot, if you want. Otherwise, could you please open an issue and mention your idea(s) there?