Add a configurable moving_average implementaion, that has a configurable
window size and is enable triggered.
Also add a moving_average_wrapper that instantiates 2 moving_average instances
(an inner and an outer) to lower the memory resource requirement for large
window sizes. Effective window size is OUTER*INNER, whereas the resources are
only OUTER+INNER. A trigger signal signals when the average is "valid".
Accompanying testebench was also implemented.
