Store time information in pmtrd and sipmrd

[gonzaponte]

At the moment the time information is implicit and depends on the parameters of the city that generated the data. This is error-prone. We don't need to store the waveform times, but this wouldn't be too bad either, as it doesn't take a lot of space. I suggest we simply store the sampling time as a node attribute for now. The waveform times can be trivially obtained as np.arange(len(wf)) * sampling_time.

#837 would introduce the parameter used by the city, which would already implement this functionality indirectly. I would rather have it duplicated and attached to the relevant information as they serve different purposes. Perhaps we will see that this is unnecessary in the future and remove it.