Test combination of number concentration and CAM-SAM conversion

[MarionBWeinzierl]

@paogorman suggests that we should do a combination of #16 and #39 , because both lead to likely suboptimal results in the TOGA II test, but it is possible that this is because of a mismatch between CAM and SAM thermodynamics with the introduction of cloud droplet numbers -- particularly with a build-up of cloud ice at high altitudes (200hPa). A conversion between variables may resolve this.