Generalize sources

Currently, agents are spawned

• according to a Poisson process (PoissonDistribution, NegativeExponentialDistribution)
• in a repetitive manner (UniformDistribution) Hence, only distributions were available.

Now, three spawning procedures are available:

• Distributions
• Different distributions over time
• Time series Therefore, the sources have been generalized. Examples:

  "interSpawnTimeDistribution" : "constant",
        "distributionParameters" : {
          "updateFrequency" : 1.0
        },

  "interSpawnTimeDistribution" : "timeSeries",
        "distributionParameters" : {
          "..." : [2,4,5,0,4]
        },