errors made by an approximate separation oracle.  In particular, the solver
will now check the output of the separation oracle against the cache and if the
cache gives a better value, even when we would otherwise not use the cache, the
cache value is used.  Similarly, we can output the truth psi vector to avoid
outputting a psi with a negative risk.  All this stuff only happens when the
cache is enabled, if its disabled then the outputs of the separation oracle are
used without any kind of modification.