Add option to add table containing lengths of pulsemaps

[AMHermansen]

Is your feature request related to a problem? Please describe.
Currently there is no quick way to find out how many pulses there are in a given event. This makes it time-consuming to do analysis involving the number of pulses. It also makes it difficult to create a torch.utils.data.Sampler which requires information of lengths of the pulsesmaps.

Describe the solution you'd like
I would like to have the option to add the number of pulses for each event of a given pulsemap to a database, this could either be in the truth table on a new pulsemapCount table. Implementation wise I think this could either be an optional argument to the extractors, or it could be an extractor by itself.

Describe alternatives you've considered
It might be outside the scope of the library, and hence end-users are responsible for implementing such a feature themselves.