Clang warnings about overridden functions

[tasmith4]

The nightly dashboards report several clang warnings about overridden functions in the master element code. @overfelt these warnings began to appear after #1022 merged, although I'm a little confused about why that PR caused those warnings. Can you take a look?

[tasmith4]

https://sierra-cdash.sandia.gov/index.php?project=Nalu-Wind

https://my.cdash.org/index.php?project=Nalu-Wind

[sayerhs]

@tasmith4 clang will complain if you add override to certain virtual functions that are overridden but not others. In this case, I suspect it is unhappy about shape_fcn and others that are overridden but don't have override directives. To remove warnings, you will either have to mark all overridden functions with override or remove override from the lines added in #1022

[tasmith4]

Fixed by #1030. Thanks @overfelt!