plugins/dac_data_manager.c: Add 32767 offset for "cf-ad9122-core-lpc"
plugins/dac_data_manager.c: Make sure that the DDS iio buffer gets destroyed
oscplot.c: Stop the plot properly before destroying it
Before a plot gets destroyed, it's drawing function should be stopped and all resources should be freed.
This fixes the bug that prevents user from successfully loading a running plot profile while an initial plot is running. The newly created plot is failing to display data properly by displaying a still image and not continuous captured data.
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