Draft: Secondary Event Stream for normalising other signals

This relates to WP2873

This merge request:

• Creates a new class that is a callback which generates a secondary stream which for a defined list of signals will look for those signal names in the events, when it finds them it will create a new named signal and divide the value by N divisors. The result is put in a new document.
• All other callbacks can be subscribed to this one.
• It works with the BEC
• It works with CSV writer assuming changes in this merge request are accepted.
• It works with the specfile writer I found in the main branch of beamline tools.

The start document is modified to add the normalised detector to the list of detectors

The divisor can be either a signal name or the name of a config parameter. This lets you use the dead_time config param of an mca for example, providied you know the name of the signal.

Pytests are included and give some idea how to use.