Replace RealLimits and AttLimits by simple numbers for upper and lower limit
RealLimits lets us choose between inclusive and exclusive limits, like 0<=x vs 0<x. However, sometimes we may want to force a value into the allowed interval. This cannot be done with exclusive limits. Therefore it is better to support only inclusive limits. The condition 0<x shall be replaced by eps<x, where it is left to the API user to choose a reasonable value for eps.
AttLimits just wraps RealLimits and one single boolean, m_fixed. This is unnecessary overhead, and should be removed first.