InequalitySystemTest: Test for integer overflow

We are testing a (relatively) trivial system with lots of inequalities of the
form -1 >= i*x[0] for i between 1 and 300. A solution would x[0]=-1. However,
choco-solver cannot find a solution, but it can do so if you we chose a smaller
bound for the variables. So this is likely some integer overflow somewhere.

Signed-off-by: Uli Schlachter <uli.schlachter@informatik.uni-oldenburg.de>
InequalitySystem: Add BigInteger API

Signed-off-by: Uli Schlachter <uli.schlachter@informatik.uni-oldenburg.de>
Add a new implementation for solving inequality systems based on oj! Algorithms

Signed-off-by: Uli Schlachter <uli.schlachter@informatik.uni-oldenburg.de>
SeparationUtility: Handle dead events

An event is dead if it does not occur on any reachable marking.

The code for calculating pure regions already handles this correctly, but the
impure case will try to add self-loops. Fix the resulting NullPointerException
by just adding a simple self-loop with weight one in this case.

Signed-off-by: Uli Schlachter <uli.schlachter@informatik.uni-oldenburg.de>