Measure class constructor

[gbellantuono]

Working on the run manager and the generation of the configuration file we found an issue.

When we generate with the configuration UI the Measure object(for example Mahalanobis), we have 2 or more parameters (mapping and mapping2). The problem is if the user doesn't put any input, we are sending 2 empty arrays. The stone soup serialiser will identify these 2 arrays as input for the Measure constructor and will initialise mapping and mapping2 as an empty array. Because they are empty they will cause a problem with the simulation and give an error.

If these 2 arrays can never be empty, I would suggest modifying the constructor and setting mapping and mapping2 as None if the inputs are empty arrays.

As a temporary solution, we don't pass empty ndarray. We are just concern that this could create issues where we want the array to be empty.