Simple custom complex class (cxsmpl) #307

This is a placeholder for an idea I had in mind since quite some time. But unless I am wrong I have not opened a ticket for this yet.

The idea is to add yet another complex typedef, in addition to std, thrust and cucomplex. We can simply create a custom complex class that works for ALL backends (c++, cuda, and possibly alpaka, kokkos, sycl).

The point is that we only really need very basic operations in complex: addition, subtraction, multiplication, division. All of these can be very simply inlined in simple code, and actually some of these ALREADY are inlined in a header for the vector complex class. So essentially this would be an extension to the scalar type (RI) of the present cxtype_v class (RRRRIIII), it should be quite easy to do.

One additional benefit of this approach would be that we can define many methods as constexpr. This would be useful for instance in the definition of hardcoded parameters (issues #23 and #39 and PR #306).