Feature/invariants init

[frejknutarlewander]

invariant/constraint initialisation has been split over three methods

• registerVars - called after the invariant is created and register input and output variables
• updateBounds - called after registerVars (when the invariant is created) and computes the bounds of the output variable(s)
• close - called when the model is closed and updates any data structures that the invariant uses (i.e. _counts for AllDifferent)

Each invariant now computes bounds of the output variables (some do tight computations, others overapproximate).

Improved unit tests for InDomain and InSparseDomain

Added PowDomain constraint to give violation when base is 0 and exponent is negative.

Added NotEqualConst constraint for IntDiv when denominator is 0.