Update remaining convert functions

[dominikbach]

Partly addresses # 609.

Fixes #610.

Contains #621 and should be reviewed after this is merged

Changes proposed in this pull request:

• generalise pspm_convert_gaze_distance to deal with any gaze unit conversions (adding metric output) and rename to pspm_convert_gaze
• refactor the core of pspm_convert_pixel2unit (which was duplicated in pspm_convert_gaze_distance) to pspm_convert_pixel2unit_core (@teddychao changed this line by updating the function names)
• remove superseded functions pspm_convert_pixel2unit, pspm_convert_visangle2sps, pspm_convert_visual_angle
• adjust tests accordingly

GUI will be adapted in a new pull request.

[teddychao]

I have approved this pull request after adding some updates to the help text for recording the contribution. I updated the GUI transferring function for gaze_convert when I was reviewing the code, but I noticed that another pull request will be required to update the GUI. So I only keep the GUI update for pspm_cfg_run_gaze_convert and do not keep checking GUI functions for other updates. I also update the document since some function names have been changed.