Pyscipopt

Added
- Continuous Integration: move from travis to github actions:
- integration with scipoptsuite and
- publishing documentation and packages.

Fixed
Changed
Removed
- CI files

Added
- add convenience function `Model.addConss()` to add multiple constraints at once

Added
- add SCIP function `SCIPsetMessagehdlrLogfile`
Fixed
- fix `Model.writeLP` method

Fixed
- check for correct stage when querying solution values (raise warning otherwise)

Fixed
- check for NULL pointers when creating Solution objects (will return None): [494](https://github.com/scipopt/PySCIPOpt/pull/494)

Added
- add `Model.getNReaders` that returns the number of available readers