Use `cpptrace` for prettier backtraces from exceptions

[ZedThree]

We have historically been reluctant to add too many external dependencies, but with CMake and spack making it easier to pull them in and build them, I thought I'd try this library for generating stack traces.

Two useful things:

1. it's very portable out of the box, so we don't need to rely on external utilities like addr2line and instead can always enable backtraces
2. we can print snippets:

#3  in checkData(double)
    at /home/peter/Codes/BOUT++/BOUT-dev/include/bout/utils.hxx:493:65
     491: inline void checkData(BoutReal f) {
     492:   if (!std::isfinite(f)) {
   > 493:     throw BoutException("BoutReal: Operation on non-finite data");
                                                                          ^
     494:   }
     495: }
#4  in operator*(Field3D const&, double)
    at /home/peter/Codes/BOUT++/BOUT-dev/src/field/generated_fieldops.cxx:480:12
     478:   Field3D result{emptyFrom(lhs)};
     479:   checkData(lhs);
   > 480:   checkData(rhs);
                     ^
     481: 
     482:   result.setRegion(lhs.getRegionID());
#5  in Blob2D::rhs(double)
    at /home/peter/Codes/BOUT++/BOUT-dev/examples/blob2d/blob2d.cxx:177:41
     176:     ddt(n) = -bracket(phi, n, BRACKET_ARAKAWA) // ExB term
   > 177:              + 2 * DDZ(n) * (rho_s / R_c)      // Curvature term
                                                  ^
     178:              + D_n * Delp2(n);                 // Diffusion term
     179:     if (compressible) {

and can even enable colour:

It also has its own signal handler we could perhaps use instead of ours (though I've not tested this out).

Lastly, this is maybe a complete enough solution that we could also remove MsgStack/TRACE/AUTO_TRACE?