get rid of global / managed runtime parameters

[zingale]

GPU compilers seem to have trouble with managed-memory runtime parameters, so we should remove them.

The idea is that we will have the current python script define a set of structs for each namespace and then a parent struct, params that has each namespace struct. E.g.:

struct hydro_params {

    int time_integration_method{};
    int ppm_type{};

};

struct gravity_params{

    int do_grav{};
    int grav_source_type{};

};

struct params {

    hydro_params hydro;
    gravity_params gravity;

};

We will then make params run_params a static member variable of the Castro class. This would allow us to access it anywhere.

If we want to simplify things, we could do something like:

auto & hp = run_params.hydro;

to just access the hydro parameters.

Currently all of the Castro runtime parameters are parmparsed in separate headers, like castro_queries.H that are all included in intiialize_cpp_runparams().

We can implement this a bit at a time. Here's an outline:

• [ ] modify parse_castro_params.py to create the struct's for each namespace and the parent struct. These can all be in a single header. This is done in PR #2688.
• [ ] ~~modify parse_castro_params.py to also create a "sync" function that initializes the runtime parameter structs by setting the value from the global version. This is temporary.~~ In the *_queries.H, also set the struct version of the parameter. This is done in PR #2688.
• [ ] Wherever we reset runtime parameters in read_params() or Castro_setup.cpp, also update the struct
• [ ] Convert all of the Castro functions to access the parameters from the new struct instead of the global header. We can do this a bit at a time.
• [ ] Once all of the code is updated, remove the global managed variables