add create_measures_table() function

[ojscholten]

A generalised functioned for creating a measured table, which accepts both a set of transaction data, a list of names of behavioural measures, and the level of aggregation of the data (individual, daily, weekly, etc), and outputs a measures table would be a great addition to the measures module.

It would also be a nice opportunity to optimise the creation process, e.g. if the duration is one measure and frequency (which uses duration in its calculation) is another, if the method were smart enough to identify and optimise this then some compute time would be saved. This optimisation is a luxury though, a naive implementation would be good in the short term.

[ojscholten]

This is a major pull towards an object oriented approach, I.e. a measures table a class with transaction data and a list of measure names as parameters in its constructor. Changing everything to OO may require substantial effort but is under consideration.