Save impedance data to a file for Cyton and Ganglion

[retiutut]

Is your feature request related to a problem? Please describe.

As a general policy, the GUI tries to save raw, unfiltered, and contiguous data to files. When we check impedance for Cyton and Ganglion though, current is injected into channels, which means we cannot collect meaningful data while checking impedance in this way. Some users have expressed they would like to analyze impedance measurements separately though.

Describe the solution you'd like

When impedance check is active, save the data to a new and separate file (e.g. OpenBCI-Cyton-Impedance-1-01-2023.txt).

Describe alternatives you've considered

This data is currently displayed in the GUI in independent widgets, one for Ganglion and another for Cyton. Users have expressed in the past though that seeing changes over time in impedance could be analyzed using Python, Matlab, etc.

Additional context

Playing back impedance data files is not required nor should it be implemented. This is an extra feature for those interested in analyzing the data offline.

Most users should use the impedance checks to adjust electrodes, stop checking impedance, then start a new recording.