Fix pspm_extract_segments

[dominikbach]

Fixes a bug detected in beta testing.

Changes proposed in this pull request:

• save events in pspm_glm
• correct computation of events in case of edge cases in pspm_glm
• add time stamp to the glm struct for troubleshooting
• correct pspm_extract_segments when time units are markers and input is a GLM

[teddphil]

I can add the description of glm to Outputs in help text if useful

[teddphil]

In pspm_extract_segments_test, at line 208, pspm_extract_segments expects that glm has the field glm.input.events, as this is used by line 158 in pspm_extract_segments. In the current version of pspm_extract_segments_test, there is no glm.input.events. I do not know if the test data shall be updated?

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This has been resolved after updating dataset. Thank you.