Enable more compiler warnings and fix them (#1538)
* Enable more compiler warnings and fix them
I've taken the default set of warnings that we found useful for other projects like wsclean and Dp3. This caused quite some warnings in casacore, particularly about using 0 instead of nullptr. There were also some issue with unused macros for old compiler versions, which have been removed.
* Removed -Wduplicated-branches as not supported by clang