Release mode doesn't run std::isfinite, isnan, isinf etc.

[danieldeidda]

When installing STIR in release mode the cxx tag uses fast math however this does not allow to use some std functions like isfininit, isnan, isinf etc. when installing with and withouth fast math math the stir_timing outputs are as follow:

so mostly fast math does make thing slightly faster but not sure that it would make any significant difference in a reconstruction