Unify channel input in PsPM functions

[dominikbach]

Summary

1. Most functions take an argument options.channel but some require a separate channel argument. This should be streamlined in the following way:

• any function that has a default channel (e.g. modality-specific last channel) should take the channel input through options
• any function that has no default (i.e. modality-unspecific functions such as pspm_extract_segments or pspm_pp) should take an explicit channel argument.

2. Specifically, the following functions are reported to have mismatching channel input:

• pspm_scr_pp: according to help, channel argument is entered via options, but the code actually expects a third argument. The GUI does not provide a third argument.
• pspm_convert_ecg2hb and pspm_convert_ecg2hb_amri do not take the same channel input