Use math.log10 instead of np.log10 when possible

In the pathloss classes we need to calculate the logarithm with base
10. If the provided distance value is a float we use math.log10 for that
and if is an Iterable (a numpy array) we use np.log10.

Previously it was always using np.log10.
Handle pathloss for small distances

Add the "handle_small_distances_bool" to the PathLossBase class. If it
is set to False then an exception is raised if the path loss is
calculated for a distance so small that it would be negative (in dB).

If handle_small_distances_bool is set to True then in case the path loss
is negative (in dB) it is assumed to be equal to 0dB.