Amici

- [all] **critical** Fix long standing bugs in solving steadystate problems (including preequilibration) (471)
- [all] Fix AmiVector constructor from std::vector (471)
- [python] Reenable Solver and Model copy constructors
- Update documentation

- replaced symengine by sympy for symbolic processing in python *which fixes several critical bugs* that were due to bugs in symengine (467)

- fixes a critical bug in objective function computation for models compiled using `sbml2amici` and `pysb2amici` that was introduced in v0.7.12
- fixes a critical bug in sensitivity computation when`model.reinitializeFixedParameterInitialStates` was set to true
- readds the python interface to the ExpData copy constructor that was inadvertently removed in 0.7.12 and streamlines the respective convenience wrapper to provide access to the full range of constructors.

- fixed a critical bug in `amici.constructEdataFromDataFrame`
- enabled multithreaded simulation of multiple experiments (requires compilation with openMP)
- modularized sbml import and added pysb import

- [python] Added numpy and python wrappers that provide a more user friendly python API
- [python] Enable import of SBML models with non-float assignment rules
- [python] Enable handling of exceptions in python
- [python] Enable nativ python access to std::vector data-structures
- [core] Provide an API for more fine-grained control over sensitivity tolerances and steady-state tolerances
- [core] Provide an API to specify non-negative state variables (this feature is still preliminary)

- Fixed python SBML import `log()` issues (412)