Fixes #670 and fixes #672 .

Changes proposed in this pull request:

1. bug in pspm_get_markerinfo #670 was caused by a faulty markerinfo structure in a file that had been preprocessed multiple times. It is unclear what caused this faulty structure; it did not appear when the file was re-imported. Added a check for the markerinfo structure in pspm_check_data to catch such cases.
2. A potential root cause for (1) is the function pspm_combine_markerchannels which creates a char rather than cell array for markerinfo.name. This was fixed.
3. problem running GLM with conditions created from markerinfo #672 was caused in pspm_get_timing, called by pspm_check_model. This check will fail because at this point, the data (and therefore, the markerinfo structure) has not yet been loaded. pspm_check_model is now adapted.
4. A bug in pspm_cfg_run_glm was removed.
5. Option to work on multiple data files for pspm_combine_markerchannels was removed, in line with Unify function output #689.