Straub fragmentation function optimization: avoid using Storage __getitem__

[slayoo]

There seem to be a significant overhead in using constructs like:
https://github.com/atmos-cloud-sim-uj/PySDM/blob/ac01b4ea5e91f3045d9ba3acbe61c828ed42d589/PySDM/dynamics/collisions/breakup_fragmentations/straub2010.py#L50
which result in clling Storage __getitem__ millions of times in simulations like the 1D rainshaft one.

A possible workaround would be to implement a method in Storage (or PairwiseStorage in this case) which would do the copying using some backend code (njitted in the case of Numba CPU backend).

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[edejong-caltech]

@slayoo Agreed we should address this for the sake of speeding up the code -- I don't feel that I understand the backend storage methods well enough to implement an njitted copying function. Is this something that you or @abulenok might be able to help with?

[slayoo]

I suggest, we do it after merging #994 (and let's merge it ASAP)

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[slayoo]

this has been addressed in #1014