Pyneqsys

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- import SymbolicSys from pyneqsys.symbolic
- provisional support for ipopt
- backend kwarg expected in callbacks pass to SymbolicSys.from_callback()

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- New solvers: 'mpmath' (requires mpmath) and 'kinsol' (requires pykinsol)
- NeqSys.solve() refactored (new signature, ``solver`` arg moved to pos 4)
- New NeqSys.solve() kwarg: attached_solver (factory which registers NeqSys instance)
- NeqSys.solve_scipy and NeqSys.solve_nleq2 was made private
- Added ChainedNeqSys
- In NeqSys.solve() arg "solver" may now be None -> $NEQSYS_SOLVER
- In NeqSys.solve() arg "solver" may now be a callable
- NeqSys.plot_series & NeqSys.solve_and_plot_series changed signature
- New methods: NeqSys.plot_series_residuals(_internal)
- Logic in NeqSys.solve_series() changed slightly

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- Fixed bug in SymbolicSys jacobian evaluation.
- Use pyodesys mechanism to dynamically use sympy/symengine/pysym for pyneqsys.symbolic

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- Use of of pre-/post-processors

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- Added ConditionalNeqSys

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- Added convenince methods: solve_series and plot_series to NeqSys