Placing parameter structs outside their respective classes

[AmroAlJundi]

If I want to use MetisReordering with the ReorderBase function, I'd need to declare the parameters object alone and use templates for it:

MetisReorder<ull, ull, val>::MetisReorderParams params;
ull *perm = ReorderBase::Reorder<MetisReorder>(params, A, {&cpu_context}, true);

As you can see we need the template types for the params object. It isn't very aesthetically pleasing. The way to avoid this is to have users pass all the members of the params object to the Reorder call which is too many:

  struct MetisReorderParams : PreprocessParams {
    int64_t ctype = metis::METIS_CTYPE_RM;
    int64_t rtype = metis::METIS_RTYPE_SEP2SIDED;
    int64_t nseps = 1;
    int64_t numbering = 0;
    int64_t niter = 10;
    int64_t seed = 42;
    int64_t no2hop = 0;
    int64_t compress = 0;
    int64_t ccorder = 0;
    int64_t pfactor = 0;
    int64_t ufactor = 30;
  };

We can also declare this struct outside the MetisReorder class. But this solution isn't "general" since there are preprocessings in which the parameters are templated (in Permute, the ordering is dependent on IDType).

I'm leaning towards declaring the params structs outside their classes. I have three reasons:

1. If the struct is not template-dependent, its declaration would be easy
2. If it is templated, then it will be the same thing as if it were inside the class as far as the user is concerned:

   SomeReorderParams<int, int, int> params;
   // vs.
   SomeReorder<int,int,int>::SomeReorderParams params;

3. Generally, if the struct is templated dependent, it is going to be created inside the class constructor itself.

The only problem is that when it was inside the class the namespacing allows the IDE to figure it out. But if the params struct and the class itself have the same prefix then it wouldn't be that bad; we can maintain the standard of ParamsTypeName = PreprocessingName+"Params"