FPE in grains

[rporter]

The following sim causes an FPE in source:trunk/source/grains.cpp. It happens at different places depending upon whether NDEBUG is set. If so, the failure is at line source:trunk/source/grains.cpp@2452#L3103, otherwise it is at source:trunk/source/grains.cpp@2452#L3123. In both cases the failure is division by zero.

blackbody 200 STE
Hden 2.0     
stop zone 1
sphere                                
Abundances H II region no qheat no grains  
grains PAH 10   
grains orion 1
no grain qheat       
element Helium abundance -0.98  
element Carbon abundance -4.14 
element Nitrogen abundance -4.2  
element Oxygen abundance -3.47  
element Neon abundance -4.4     
element Sulphur abundance -5.06  
element Chlorine abundance -6.88   
element Argon abundance -5.64  
element Iron abundance  -5.5   
element Boron off                 
element Beryllium off    
element Scandium off            
element Vanadium off           
element Copper off         
element Cobalt off  
element Nickel off    
element Zinc off   
element Manganese off  
element Titanium off 
turbulence 3km/s 2

[rjrw]

Minor wiki formatting finesses.

[peter]

This sim is quite pathological since there is absolutely no ionization anywhere. This means that the ionization and recombinations for the grains are all zero, causing a division by zero down the line. Reducing the priority to minor.