docs and traj1d/2d: make it clear which methods work on 1d and 2d datasets

[gauteh]

Many of the methods require the dataset to be in a certain layout. E.g. condense_obs only works on 2d datasets. This should be clear in the documentations, and it should either raise a useful error, or it should be a no-op when that is safe (e.g. condense_obs should perhaps be a no-op on 1d?).