Change the handling of element_composition

[gasevic]

Currently, the handling of the element composition often leads to structures with only the given minimum number of atoms.

For example with the setting element_composition = "Sr:4-8" most structures will have exactly 4 Sr atoms since they are set to the minimum amount if during the generation fewer atoms are generated which happens often. Ideally, the amount of Sr should be set to a random number between 4 and 8.

[marcelmbn]

Thanks for your feedback!
I'll implement it in such a way that the element_composition is not longer only "corrected" until it reaches the upper or lower limit (effectively, most of the time, the latter is happening), but instead, a random number within the interval is chosen.