Support for importing exported segments from Biopac AcqKnowledge (*.mat)

### Feature Description

Currently, the function `pspm_get_acqmat` does not support .mat files **exported as segments** from Biopac AcqKnowledge. 

### Feature Request:
Update `pspm_get_acqmat` to support exported segmented .mat files.

### Relevant Code

`pspm_get_acqmat`  around line 63
