Source spawn distribution

This branch gets rid of the old spawn algorithm und uses the new one.

By default, the new spawn algorithm works like the old one: constant inter-spawn times.

The new algorithm can be configured to use different spawn distributions, e.g.

      "sources" : [ {
        "id" : 1,
        "shape" : {
           ...
        },
        "spawnDelay" : -1.0, /* this one does not exist anymore */
        "interSpawnTimeDistribution" : "org.apache.commons.math3.distribution.ExponentialDistribution",
        "distributionParameters" : [ 1.4 ], /* mean for exponential distribution: 1.4 s */
        "spawnNumber" : 1,
        ...

The default distribution is: org.vadere.state.scenario.ConstantDistribution

There is an automatic migration from "not a release" to version 0.1 which drops the spawnDelay property.