Consolidate naming of LinearPSMDataset OnDiskPSMDataset

[gessulat]

This issue stems from this discussion.

MSAID's streaming branch introduces the OnDiskPSMDataset class which is enables the chunk-wise streaming of a PSMDataset and returns this chunks as LinearPSMDataset. To make this relationship more explicit and clearer to developers it would be nice to consolidate the names. For example InMemoryPSMDataset and OnDiskPSMDataset.