[gaml] add a parallel facet to the loop statement and a parallel parameter to container operators

[lesquoyb]

Is your request related to a problem? Please describe.
In the same way that there's a parallel facet for the ask statement or the grid declaration, there could be one for the loop statement when it is not an until loop.
This could improve performances in many models.

Additionally, we already know that the container operators like collect, where etc. are more performant than going through loop statement, so maybe we can check to parallelize them too as there are parallel streams available in java.

Describe the improvement you'd like
Adding an option to run loops and container operators in parallel in gaml, add a mention to it in the optimizing models page