Remove unneeded numpy.ndarray to list conversion

Decoded data will be typed to numpy.ndarray after windowing anyhow, the conversion is completely unnecessary
Fix #5

Mapping from k to F:
F = k * Fs / N, for k = 0, ..., N/2

Mapping from F to k:
k = F * N / Fs, for F = 0, ..., Fs/2
Fix type inference for function parameters
Fix #6 frequency magnitude computation

Magnitude = Re * Re + Im * Im
Fix import error

Miss changing of numpy to np as change of import