Add LHCb::span, LHCb::make_span to STLExtensions
- LHCb::span is a small facade to GSL::span to ensure that the boundary checking of GSL::span is only enabled in debug builds.
- add LHCb::make_span to avoid having to specify more types than strictly necessary (note: GSL::make_span isn't available in the version of GSL which ships with LCG91
- add a small unit test to test all features in STLExtensions.h