Add an example on how to use a component reading to create a new source

[nvaytet]

When we have choppers that block almost all neutrons with very small openings, it is wasteful to just keep increasing the number of neutrons in the source to get good statistics.

We can add an example in the docs on how to use the data read at the chopper to make a new neutron source, place it just before the chopper, and sample many neutrons in that source.

[nvaytet]

This turns out to b a bad way of doing things (see #131).

Instead this is done differently in #136