Make the input arguments of FastScape::initialize_fastscape() const. #6492
Description
As far as I can see, the arguments to FastScape::initialize_fastscape() are all input arguments that are not modified in the function. Our convention is to mark such argument as const, to make their intent ("input") clear to the reader and the compiler. I would fix this right away, but that would step on #6472.
@Djneu If you wanted to address this in a follow-up PR, feel free to!