oscplot.c: Overwrite an existing mat file instead of appending data
plugins/dac_data_manager.c: Refactor analyse_wavefile() function
 - make sure that .mat files saved by the iio-oscilloscope can be loaded by into the DAC output buffer
 - at this point only .mat files scaled to +-1 can be loaded since the unscaled ones contain data in single precision format that will not be read by the analyse_wavefile() function