Add phase measurement to spectroscopy mode

[DuzaBF]

Device returns only impedance magnitude in bis mode. It would be useful to also have the impedance phase. It seems that CMSIS DSP library does not contain a function to calculate phase from complex values. https://github.com/ARM-software/CMSIS_5/issues/293 It can be implemented via atan2 function with conversion to float (by @tachen-cs).

for (n = 0; n < numSamples; n++) {
pDst[n] = atan2(pSrc[(2n)+0] + pSrc[(2n)+1]);
}