add fixed displacements as new rattle method

[QuantumChemist]

following a discussion with @kaueltzen , I found it would be a good idea to add the fixed displacement method from hiphive that Björn uses in his Bachelor thesis to the rattling methods.

[vlderinger]

Are there parallels / overlaps / synergies with @nfragapane's structure generation code here?

[JaGeo]

@vlderinger yes, this will just be an additional option for structure generation