Improving `pspm_help`

[dominikbach]

Feature Description

1. Change pspm_help such that the second output argument, arguments is a struct with field names corresponding to the argument names of the target function.
2. Add field "References" to the first argument. This should be a cell array of char, in which each char vector corresponds to oen reference. (pspm_glm is a good test case).
3. Describe the rules by which function help is parsed. Currently, this throws errors for most functions but it is hard to fix these.
4. If possible, parse (or remove) matlab help references (as in pspm_pupil_correct_eyelink); if too complicated this can also be done manually on a case-by-case basis.