Support min and max expression operators of arbitrary size (#67)

* Support min and max expression operators of arbitrary size

There was an explicit implementation for various minN and maxN
derived counter expression operators. This replaces all those
implementations with a single min amd max implementation that
works for any arbitrary value of N.

* Did the same for the sum operator

sum and sumN, with N being any positive value >= 1 is now
supported

Author: jcortell68