Add tests for particle correlations

Ideas for additional tests:

• a square lattice of boxes with density 1: should behave like a homogeneous layer
• a rectangular (2:1) lattice with 2-particle basis vs square lattice with 1-particle basis
• slicing in combination with several layouts (incoherent sum) - some suspicion about the successive averaging in ReSample.cpp:setAvgeMaterials

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changed title from Add tests for dense particle layouts to Add tests for particle handling

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changed title from Add tests for particle handling to Add tests for particle correlations

mentioned in issue #584 (closed)

marked this issue as related to #613

mentioned in issue #613

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mentioned in issue #617

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marked the checklist item a square lattice of boxes with density 1: should behave like a homogeneous layer as completed

marked the checklist item a square lattice of boxes with density 1: should behave like a homogeneous layer as incomplete

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