Support segmented AcqKnowledge imports

[4gwe]

Fixes#848

Changes proposed in this pull request:

Support segmented AcqKnowledge MAT imports with non-zero start_sample.

The import no longer aborts for non-zero start offsets. Instead, PsPM warns about the offset and stores start_sample together with its unit in sourceinfo.

[dominikbach]

Quick clarification questions.

1. Does Acknowledge allow digital channels recorded as time stamps? And if yes, are these time stamps recorded wrt to the original recording start? Should they then be corrected wrt to wave recording start?
2. Is the offset the same for all wave channels, or could they have different offset?

[dominikbach]

see my questions.

[4gwe]

Quick clarification questions.

1. Does Acknowledge allow digital channels recorded as time stamps? And if yes, are these time stamps recorded wrt to the original recording start? Should they then be corrected wrt to wave recording start?
2. Is the offset the same for all wave channels, or could they have different offset?

1. When exporting in the MATLAB format, all channels need to be represented as waveform data, including events, so events have to be converted to a waveform channel before export. AcqKnowledge 5/6 can also export events separately using "Export for MATLAB/Octave with Events", which creates a separate Markers.m file that can be used to import the markers into the MATLAB workspace. I could not find sufficient information to include support for this in pspm_get_acqmat.

2. For wave channels, the *.mat contains a single 'start_sample' for the entire data array, so the offset for all exported channels.

[dominikbach]

Sounds good to me.