`HECGeometry` sets a number of its member variables in the `intitialize`
method based on values retrieved from the `HECDetectorManager`. Some of
these variables remained uninitialized in the constructor though.
Switched to using brace-initialization syntax to default-initialize all
member variables directly in `HECGeometry.h`. Some member variables are
set properly during the `initialize` method still though. Should fix
Coverity Defect 115452.


Former-commit-id: d22a7be11db273579739aaef474d64105c375ace