Streamline channel input

[dominikbach]

Fixes #679 and fixes #600 .

Changes proposed in this pull request:

• channel input in all functions that act on channels or use marker channels is unified.
• any function that has a default channel (e.g. modality-specific last channel) takes the channel input through options (for first level models, through model)
• any function that has no default (i.e. modality-unspecific functions such as pspm_extract_segments or pspm_pp) take an explicit channel argument.

List of functions that work on one channel with default

1. Changed in this PR

   pspm_convert_ecg2hb
   pspm_convert_hb2hp
   pspm_convert_ppg2hb
   pspm_find_sounds
   pspm_resp_pp
   pspm_scr_pp
   pspm_split_sessions

2. Already in line with this convention and therefore unchanged:

   pspm_convert_ecg2hb_amri
   pspm_dcm
   pspm_emg_pp
   pspm_glm
   pspm_gaze_pp
   pspm_sf

List of functions that potentially work on multiple channels with default

1. Changed in this PR

   pspm_convert_area2diameter
   pspm_convert_au2unit
   pspm_pupil_correct_eyelink

2. Already in line with this convention and therefore unchanged:

   pspm_convert_gaze
   pspm_pupil_pp
   pspm_find_valid_fixations

List of functions that work on one channel with no default

All in line with this convention and therefore unchanged:

pspm_extract_segments
pspm_pp
pspm_remove_epochs
pspm_interpolate

[teddychao]

Hi @dominikbach I made some minor comments and questions above. I do not think there is anything that is critical, just to confirm with you my understanding from the changes is correct. I think this PR can be merged if you are happy. Thanks