Improve filtering on the RF and AX.25 level

New features

• Instead of a rational resampler, the first decimation stage is performed using an LPF and decimation. With the LPF and the known output sampling rate, the transition with of the LPF is adjusted to be as relaxed as possible, significantly reducing the CPU overhead.

Fixes

• As we now have enough CPU resources, the intermediate bandwidth after the first stage decimation has been increased to mitigate the issue discussed at #159 (closed) (closes #159 (closed))
• Due to many false positives, the AX.25 decoder now requires at least two consecutive AX.25 flags to be received.