Synpp

- Fix: Gracefully close zmq Context and Socket at progress server close
- Add global stage aliases to provide drop-in replacement for existing stages
- Add *externals* support
- Bugfix: rollback unintended change of behaviour when running directly from the
CLI/YAML for running required stages (instead of loading from cache).

- Information of which stages are requested in config now available in context,
which allows improved handling of optional stages
- Decorated stages improved and better documented

- Running functions as stages now possible with synpp.stage decorator
- Added a wrapper class for interactive use, e.g. in Jupyter
- Devalidation less sensible for class-stages
- Added option to load required stages directly from cache without devalidating

- Bugfix: hashing of input files

- Allow hierarchical configuration options
- Devalidate stages if code has changed

- Make deletion available for write-protected directories in Windows