Long functions in modmesh::inout::Gmsh should be defined outside the class declaration

[yungyuc]

Some long functions, e.g., Gmsh::is_valid_transition() (and more), should be moved outside the class declaration and header file:

    bool is_valid_transition(const std::string s)
    {
        if (last_fmt_state == FormatState::BEGIN)
        {
            return s == "$MeshFormat";
        }
        else if (last_fmt_state == FormatState::META_END || last_fmt_state == FormatState::PYHSICAL_NAME_END)
        {
            return s == "$PhysicalNames" || s == "$Nodes";
        }
        else if (last_fmt_state == FormatState::NODE_END)
        {
            return s == "$Elements";
        }

        return false;
    }

(In this specific case, the argument should use a const reference const std::string & s to save a copy.)

This issue tracks the work to refactor the Gmsh class.

[ThreeMonth03]

@meng5113 Do you want to solve this issue? I think it may be suitable for you.

[ThreeMonth03]

@KHLee529 Are you interested in refactoring the code?