Returns the restrictions which were used to generate the match.
optmatch_restrictions(obj)A list of min.controls, max.controls and either
omit.fraction or mean.controls.
An optmatch object
If mean.controls was explicitly specified in the creation of the
optmatch object, it is returned; otherwise omit.fraction is given.
Note that if the matching algorithm attempted to recover from initial infeasible restrictions, the output from this function may not be the same as the original function call.