ENH: support true/false, yes/no values for FOAM_ABORT (#1896)
- consistent with FOAM_SIGFPE etc. - centralize code as error::useAbort() static function to avoid scattering the logic throughout the code. ENH: also accept "0" and "1" string values for Switch - not the normal path for Switch input (eg, from a dictionary), but consistent with bool definitions and simplifies string parsing. This means that `FOAM_SIGFPE=1 application` will now also work.