Add return_tiles argument

[rubencalje]

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[rubencalje]

So a lot of changes, @martinvonk . Can you explain what the purpose of these changes is? And I think we should use type hints everywhere in NLmod, or no type hints at all (which I like more).

[martinvonk]

I tried to add some code to use the old _download_and_combine_tiles function to merge the separate tiles. This function used a memoryfile thingy, and the merge function instead of merge_arrays which might be a solution to #429.

And yes, I added some type hints 👼. Originally because I was confused about the expected input for some functions. Later more, because I thought it is nice to be explicit in the function call which specific version (strings) need to be provided for an AHN request. Btw, there are already some type hints in NLmod. I'm always in favor of more documentation instead of less.