replace approximation for expm1 by std c++11 function
replace approximation for expm1 by std c++11 function

SKIRT/SKIRT,raesjo/SKIRT,SKIRT/SKIRT,SKIRT/SKIRT,raesjo/SKIRT,raesjo/SKIRTcorona,raesjo/SKIRT,raesjo/SKIRTcorona,raesjo/SKIRTcorona