Carry behaviour alignment into NWB files

[celefthe]

Follow-up to #128. align writes /timestamps_aligned to HDF5 only. NWB files link DeltaFSeries.timestamps to the ISO /timestamps dataset and keep the first raw frame as session_start_time, so an aligned session's NWB file carries no behaviour alignment. Updating an existing NWB file in place is not possible

Suggested fix:

• io.read_timestamps_aligned: for an NWB path, resolve the linked _preprocessed.h5 (move register.linked_preprocessed_path into io, keep a re-export) and read /timestamps_aligned from it. This gives process regression, smooth, zscore and regions NWB support with no further changes.
• align: accept an NWB path and resolve the linked _preprocessed.h5 the same way, writing to that file.
• export nwb: when the linked HDF5 has /timestamps_aligned, set session_start_time and timestamps_reference_time from the session_start_time attr (localised with tzlocal if naive) and replace DeltaFSeries.timestamps with the float64 aligned array before NWBHDF5IO.export. Do the same for RegisteredSeries when its linked _registered.h5 is aligned. Set the fields through container.fields[...] plus set_modified()