melissa: build fix

The previous iterative-stats package didn't produce any output. Also updated melissa to the latest commit.

A problem I've run into is that the heat-pde example requires mpirun on the environment, but fails to launch with openmpi>5 (as apparently mpirun -- <foo> is only valid in openmpi@4). I guess relying on a openmpi globally available is good enough, but could be wrapped as well.