Baseband sample rate input
For the devices capable of an arbitrary sample rate and option is added to enter the baseband sample rate (hence bandwidth) instead of the device sample rate at a given decimation or interpolation factor. This can be convenient when a precise bandwidth is desired so there is no need to calculate the resulting device sample rate.
Devices concerned: BladeRF 1 and 2, HackRF, LimeSDR, PlutoSDR, RTL-SDR, XTRX.
This implements issue #310
RTL-SDR fix
At preset load the gain was not properly set and one needed to change the gain back and forth to get the appropriate value. For some reason related to RTL-SDR the gain has to be set last as some other settings can influence it. This fixes issue #321