Constraints which invoke recoverable error

[bjodah]

During integration the stepper may try invalid values. Some solvers (e.g. CVode) has support for "recoverable errors". The plan is to let the user specify constraints:

>>> SymbolicSys([(y0, -k*y0), (y1, k*y0)], x, [k], constraints=[y0>=0, y1>=1])

this will require support in Lambdify, see symengine/symengine#1087 and bjodah/symcxx#14