NetCDF package: add version 4.6.3 and update naming (#10786)
NetCDF package: add version 4.6.3 and update naming (#10786)

* Add NetCDF version 4.6.3
* Naming scheme for artifacts >= 4.6.2 has changed. Add a
  url_for_version function to generate the appropriate URLs
iulian787/spack,iulian787/spack,LLNL/spack,LLNL/spack,iulian787/spack,LLNL/spack,LLNL/spack,iulian787/spack,iulian787/spack,LLNL/spack