Introduce higher and lower bounds for the number of instances (#178)

* Introduce higher and lower bounds for the number of instances

Reduce the number of instances to the min_instances if it is lower and
to the max_instances if it is higher. -1 for either of those means there
is no lower or upper bound.

In addition, terminate the operator when there is a nonsense in the
configuration (i.e. max_instances < min_instances).

Reviewed by Jan Mußler and Sergey Dudoladov.