This fixes "Conditional jump or move depends on uninitialised
value" error issued by valgrind. The atof() call resulted in the
valgrind error when the data is non-numeric. This fixes the error
by using std::stringstream to parse the data instead of atof().

This should fix most of the valgrind errors in the chemistry tests.

Change-Id: I9dfbcf74524d8862d09ded5ba1a85d5dc9d160b3