move to native R parallel functions

[florianhartig]

remove

doParallel,
foreach,

to reduce dependencies

[florianhartig]

This was partly addressed in de393ed

but the pure parallel version had a lot of export problems, so I changed back to foreach for now

[florianhartig]

@EttnerAndreas, could you have a look at this?

[florianhartig]

OK, it would be really good to get this thing done, and I think it's a nice contained project. Andreas, maybe you could give it a try, ideally in your fork?

See my first try in https://github.com/florianhartig/DHARMa/pull/179/files but it doesn't make sense to continue this, as we have since changed the runBenchmark function.

What we would like to have is that the function behaves exactly as before, but with native R parallelization

[EttnerAndreas]

alright, I will see what I can do. I will use my fork and let you know how it is going