Add length of recording to spatial_firing data frame

[klaragerlei]

Is your feature request related to a problem? Please describe.
It would be useful to have this (in seconds) so the position data doesn't have to be loaded later on for combined analyses.

Describe the solution you'd like
All to the df during the pipeline.

Describe alternatives you've considered
Use mean firing rate and number of spikes to estimate.

Additional context
I need this for the review (I'll just estimate) and I think it could save the next person some time potentially if we added that so I don't want to forget to do it next time I touch the pipeline.