Collection of minor issues and enhancements

[maxmargraf]

This is a list of non-critical issues that improve the usage of pycomlink.

• pycomlink/pycomlink/processing/k_R_relation.py:87 throws a zero division warning that should be silenced: RuntimeWarning: divide by zero encountered in divide
  R[~nan_index & (A >= 0)] = (A[~nan_index & (A >= 0)] / (a * L_km)) ** (1 / b)
• remove tqdm in

  pycomlink/pycomlink/processing/wet_dry/nearby_wetdry.py

  Line 53 in 94b612c

  for i, cmlid in tqdm(enumerate(cml_ids)):

• check nan behavior of https://github.com/pycomlink/pycomlink/blob/master/pycomlink/processing/wet_dry/cnn.py should be checked so it is compatible with Polz et al. (2020)
• resolve rounding issues in radar reference grid
• ...