BBPSSWProt in the ProtocolZoo

[LBacciottini]

Implemented and incorporated the BBPSSW distillation protocol from https://arxiv.org/abs/quant-ph/9511027 into the ProtocolZoo.

Created a new file ProtocolZoo/distillation.jl that is now included in ProtocolZoo.jl, following the same style that was used for swapping.jl. This way, additional well-known distillation protocols (e.g. DEJMPS) can be added in the future to distillation.jl with minor/no changes to other files.

Tests for new functionalities implemented in the new file test/test_protocolzoo_distillation.jl

Note: I didn't edit the CHANGELOG file to avoid conflicts.

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