Re #6314. Add a method to calculate the time-averaged value of a log

...in a given range of time(s).
This is needed for the SumEventsByLogValue algorithm.
Re #6314. Add columns for normalisation & other logs.

Columns for nomalisation are: monitor counts, proton charge & time.
Columns will also be added to give the average value of other time-series
logs for each step.