Fix valgrind error in vtkBlueObeliskDataParser

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
Merge topic 'fix-blue-obelisk-data-parser-valgrind-error' into master

6987a855 Fix valgrind error in vtkBlueObeliskDataParser