Io buffer and loss rename

[freddieknets]

Description

Three quick but important changes:

• If no io_buffer is specified at the tracker initialisation, but (some of) the line elements have a io_buffer specidied, this one is automatically assigned to the tracker (it is checked that they all point to the same io_buffer)
• Renamed allow_backtrack into allow_loss_refinement
• Drift slices were missing allow_loss_refinement=True

Checklist

Mandatory:

• I have added tests to cover my changes
• All the tests are passing, including my new ones
• I described my changes in this PR description

Optional:

• The code I wrote follows good style practices (see PEP 8 and PEP 20).
• I have updated the docs in relation to my changes, if applicable
• I have tested also GPU contexts

[giadarol]

already merged