fix: add elmerfem channel

When the WSL image is installed, it cannot access the flake inputs
anymore, so Elmer has to be installed via a channel.

refactor: use the Elmer overlay

feat: tell flake and system to use elmer cachix and use elmer's overlay

fix: expose mpirun to the user

refactor: change rundir

refactor: change repo to ElmerCSC

fix: add correct channel

fix: change wrong username

fix: change "nixos" to "elmer" in tests and documentation

fix: make elmer into a proper module

This fixes the flake check not passing