Coregistration for CTF data without headshape points + Improvements

[cgohil8]

We sometimes have sMRI fiducials we've recorded. We want to match these to the MEG HPI fiducials rather than using the hardcoded landmark sMRI fiducials.

Changes:

• Options to read HPI fiducials from fif info when you have CTF data.
• Option to specify MNI fiducials.
• Added function to extract polhemus data from pos files.
• Suppressed warnings when saving coreg.html.
• Added warning if _trans in preproc filename to let the user know the coreg plot might be incorrect.
• Updated examples.
• Refactored code.

Example usage for saving MNI fiducials:

from osl import source_recon

config = """
    source_recon:   
    - extract_polhemus_from_info: {}
    - save_mni_fiducials:
        filepath: data/fiducials/{subject}_smri_fid.txt
    - compute_surfaces:
        include_nose: False    
    - coregister:
        use_nose: False
        use_headshape: False
"""

source_recon.run_src_batch(
    config,
    src_dir=src_dir,
    subjects=subjects,
    preproc_files=preproc_files,
    smri_files=smri_files,
)