Updates to pspm_pupil_correct_eyelink and pspm_pupil_correct

[4gwe]

Fixes #887 and #888 .

Changes proposed in this pull request:

Fix cardiac, gaze, pupil, and channel preprocessing workflows

• pspm_cfg_convert_cardiac
  Added an ecg2hp configuration that performs ECG → heartbeat → heart-period conversion. Heart-period limits are now limit_lower and limit_upper options.

• pspm_cfg_ecg_editor
  Fixed the limits configuration tag to limits and changed the default correction factor from 2 to 1.

• pspm_cfg_pupil_correct
  Removed the obsolete screen-size configuration fields because gaze data must now be converted to millimetres before pupil correction.

• pspm_cfg_run_convert_cardiac
  Updated cardiac conversion jobs to use the new limit fields (limit_lower , limit_upper) and correctly map heart-rate settings. The ECG-to-heart-period workflow now runs ECG-to-heartbeat followed by heartbeat-to-heart-period conversion.

• pspm_cfg_run_ecg_editor
  Updated the configuration mapping to use the nested limits structure and represent missing artefact epochs with an empty value.

• pspm_cfg_selector_channel_action
  Clarified that the replace action replaces the processed channel.

• pspm_convert_au2unit
  Corrected area-to-diameter conversion by using pspm_convert_area2diameter and fixed channel-data assignment. Added status checks so failed unit conversions or channel writes return correctly.

• pspm_convert_ecg2hb_amri
  Passed the write options to pspm_write_channel so the AMRI conversion message and output-channel behavior are preserved.

• pspm_convert_gaze
  Added validation for unsupported degree-to-metric conversions and improved exact unit matching. Updated channel replacement behavior for screen-position outputs.

• pspm_convert_hb2hp
  Replaced the nested limit structure with limit_lower and limit_upper. Improved error handling for channel loading and writing.

• pspm_convert_ppg2hb
  Added filename validation, corrected missing-value detection, and improved HeartPy status messages. Updated channel writing and return-status handling.

• pspm_expand_epochs
  Added an early return when option check fails.

• pspm_load_channel
  Updated small function documentation formatting issus.

• pspm_load_gaze
  Initialized the eye suffix to prevent an undefined variable when no eye identifier is provided.

• pspm_options
  Removed obsolete pupil-correction screen-size defaults and fixed the check for an existing message option.

• pspm_pupil_correct
  Changed the function to accept a PsPM filename or data structure and load the pupil and gaze channels internally. Added validation requiring gaze channels to be in millimetres.

• pspm_pupil_correct_eyelink
  Removed internal pixel-to-millimetre conversion and the related screen-size options. The function now requires pre-converted gaze data and delegates correction to the updated pspm_pupil_correct.

• pspm_write_channel
  Updated replacement-channel selection to match by channel type rather than requiring both channel type and units.

• Tests and documentation
  Updated the affected unit tests and current test-status documentation to cover the revised options, validation, conversion, and channel replacement behavior.