Skip duration plots for single duration

[hagenw]

Closes #73

This skips creating a file duration plot, if all files have the same duration and simplifies the resulting string to e.g.

duration distribution: each file is 0.1 s

[ChristianGeng]

The feature ios useful: it does not make sense to plot a uniform distribution.

There are only minor concerns, most importantly about how intuitive the distribution string is for users.

I have given am approval anyway, as there is fairly little to do before this can be merged.