Max spawn number

This branch adds a maximum spawn number to the SourceAttributes.

Use case:

	 * This attribute can be used together with non-constant distributions. For
	 * example, consider an exponential distribution. The times of events are
	 * random. How to ensure, that exactly 10 elements are spawned? Solution:
	 * Set the {@link endTime} to 1e9 and this attribute to 10.