Pyscipopt

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

Added
- add evaluation of `Expr` in `Solution`.

Added
- add more SCIP functions: `getNSols`, `createPartialSol`

Fixed
- consistent handling of filenames for reading/writing
- fix error when no SCIPOPTDIR env var is set on Windows

Added
- add more SCIP functions: `getNTotalNodes`, `getNIntVars`, `getNBinVars`

Fixed
- `getTransformedVar` now uses `SCIPgetTransformedVar` instead of `SCIPtransformVar` which captures the variable