Multiple bug fixes

[dominikbach]

Multiple bugs and improvements identified during beta testing.

• fixes bugs in channel index handling
• fixes bugs in pspm_init (duplicated path removal, no warning if only required paths are already on the path)
• fixes a bug in a new functionality of pspm_dcm/pspm_dcm_inv (argument value inf is not recognised as int by pspm_options)
• fixes a bug in pspm_extract_segments (normalisation with missing values, sr for glm)
• fixes a bug in pspm_glm (wrong sample rate)
• improves pspm_trim (use pspm_load_channel and fix help text)
• improves pspm_glm (remove unnecessary and impossible orthogonalisation in edge case)
• catch an edge case in epoch definition (negative epoch onsets) in pspm_get_timing and pspm_trim
• improve interpolation in pspm_convert_hb2hp and pspm_resp_pp by using nearest neighbour instead of linear extrapolation (including a new regression test)
• fixes a bug (missing index for a vector) in pspm_multi2index
• improve pspm_pupil_pp by using a different criterion for the decision on whether to combine pupils

[teddychao]

I fixed the issue of pspm_init in PR 746.