merge _prob_params and Castro's _cpp_parameters

[zingale]

Right now, we have a different make rule / mechanism for generating the problem parameters (looking that the problem's _prob_params file) than we do for all of the other Castro parameters. The parameter files are basically the same, so we should just merge this into a single process, all handled by the main Castro parameters.

This will make #2685 easier

[zingale]

actually, this is a little bit tricky, since the _prob_params support arrays.