Pull request #128: Handled nullptr filename in gputil::ApiException

Merge in NAV/ohm from bugfix/gpu_api_exception_default_filename to master

* commit 'e60dbd669f24bce59e79349ad166f51e88ee27ef':
  Fixed comment about null filename in gputil::ApiException.
  Removed unnecessary DEFAULT_FILENAME from gputil::ApiException.
  Added __FILE__ and __LINE__ to all calls to gputil::ApiException, also handled filename being nullptr in the constructor.