Buildsystem improvement

The current buildsystem has some issues:

• cmake "Superbuild" is not well suited to building many interdependent projects with potentially complex policies,
• cmake is not well suited to installing python,
• installing python libs to a prefix does not work well,
• mixing python versions is a nightmare.

So, in order to resolve that we should:

• as a stopgap, validate the improved SuberBuild ( !214 (merged) ),
• improve PDI in docker experience (see #138 (closed) ),
• use setup.py/distutils to package python libs instead of cmake ( #149 (closed) ),
• integrate PDI in the spack default repository ( #151 (closed) ),
• provide a self-contained PDI distribution using spack ( #150 (closed) ).