HpMultiGrid.H: add default case to switch statements

[lucafedeli88]

This PR adds the default case to the two switch statements in HpMultiGrid.H, in order to respect the following clang-tidy rule:

• bugprone-switch-missing-default-case

With this PR, almost all the checks of the bugprone-* family in clang-tidy v20 would pass, except:

• bugprone-easily-swappable-parameters
• bugprone-implicit-widening-of-multiplication-result
• bugprone-narrowing-conversions
• bugprone-unchecked-optional-access

Of these, the first three are probably too annoying to enforce in HiPACE++, while I think that the last one can be useful.

• Small enough (< few 100s of lines), otherwise it should probably be split into smaller PRs
• Tested (describe the tests in the PR description)
• Runs on GPU (basic: the code compiles and run well with the new module)
• Contains an automated test (checksum and/or comparison with theory)
• Documented: all elements (classes and their members, functions, namespaces, etc.) are documented
• Constified (All that can be const is const)
• Code is clean (no unwanted comments, )
• Style and code conventions are respected at the bottom of https://github.com/Hi-PACE/hipace
• Proper label and GitHub project, if applicable

[AlexanderSinn]

Here, system_type is only supposed to be 1, 2 or 3 so an amrex::Abort() would be more appropriate. The return 0 is just to get rid of compiler warnings.

[lucafedeli88]

Here, system_type is only supposed to be 1, 2 or 3 so an amrex::Abort() would be more appropriate. The return 0 is just to get rid of compiler warnings.

What about this solution?

    static constexpr int get_num_comps_acf (int system_type) {
        switch (system_type) {
            case 1:
                return 1;
            case 2:
                return 2;
            case 3:
                return 0;
            default:
                amrex::Abort(std::to_string(system_type) + " is not a valid system_type!");
        }
        return 0; //unreachable
    }

[lucafedeli88]

The content of this PR has been included in #1303