Use reference density in path integration
Added new cross exec to dry thermo
Fix for faulty calculation of the (cloud) cover
Throw warning instead of error when providing illegal input to crosslists
Added crosses to boundary_surface
Towards unified names for the cross sections
Merge remote-tracking branch 'thijs/thijs' into stats
Pass the stats functions by value and not by reference
Merge remote-tracking branch 'bart/newcross' into stats

Conflicts:
	src/thermo_dry.cxx