Test correlator based on synthetic data

[kif]

They can be generated from molecular dynamics using:
https://github.com/aryabhatt/xpcs-baseline

[kif]

Using synthetic data would also allow to tune unit-tests so that they last for a second each and allow more tests to be run in a dedicated time (~few minutes)

I believe a benchmark-suite should be separated from the test-suite and used only for performance assessment.