Pyscipopt

Added
- add support for concurrent optimization
- note that SCIP needs to be linked to a TPI (task processing interface) to use this feature
- SCIPsolverConcurrent implementation from issue 229 by TNonet in 535
- fix action to also run on external PRs by mattmilten in 536
- fix concurrent solve test by mattmilten in 537

Added
- add SCIP function `getPseudoBranchCands`

Fixed
Changed
Removed

Added
- add SCIP functions `getNSolsFound`, `getNSolsFound`, `getNLimSolsFound` and `getNBestSolsFound`

Fixed
Changed
- documentation build parameter EXTRACT_ALL = YES

Removed

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