Fix an incorrect unit scaling bug in get_frequency_hertz().

Fix a bug in get_frequency_hertz() which was causing it to return incorrect values, depending on which frequency unit was currently active on the device's channel settings.

Add get_frequency() which returns the frequency in whatever unit the device's channel is currently set on. If you'd prefer to always get the response in Hz, use get_frequency_hertz() instead.

Add a bunch of unit tests for some of the functions in the command module. More unit tests will be added soon.