Allow additional, arbitrary startup routines

[eschnett]

This would be used to register the "slicing condition" with CoordGauge.

[ianhinder]

How would such a feature work? Would you specify a string to Kranc which was the verbatim C-code to call in a startup function? Maybe we need a generic hook mechanism.

[ianhinder]

Maybe you can supply the name of a .c file which should be added to the thorn, and allow functions from this to be scheduled. This would allow you to custom-write any code that you needed; the only limitation would be that the code had to be called as a scheduled function.