Check if AutoPasContainer::getNumberOfParticles() is still broken

[FG-TUM]

Describe the bug
The code for AutoPasContainer::getNumberOfParticles() manually counts molecules instead of using the counters of AutoPas. A ToDo there suggests that this is done due to a bug but it is unknown if this still applies. Needs some investigation!

Link to the code:

ls1-mardyn/src/particleContainer/AutoPasContainer.cpp

Lines 405 to 412 in 451c70d

	unsigned long AutoPasContainer::getNumberOfParticles(ParticleIterator::Type t /* = ParticleIterator::ONLY_INNER_AND_BOUNDARY */) {
	unsigned long count = 0;
	for (auto iter = iterator(t); iter.isValid(); ++iter) {
	++count;
	}
	return count;
	// return _autopasContainer.getNumberOfParticles(); // todo: this is currently buggy!, so we use iterators instead.
	}