Feature field average windowing

Extended the fieldAverage FO to include an 'exact' windowing method.

In earlier versions the window option generated a pseudo/approximate window average. The type must now be specified by the windowType entry, e.g. for backwards compatibility, use :

windowType  approximate;

and for the new exact window average

windowType  exact;

Note that the exact method requires all instances of the field to be cached within the windowing interval, leading to increased memory usage, and also need to be written to enable a clean restart. To suppress writing of all intermediate field files (disabling smooth restart) use the option

allowRestart yes;

mentioned in commit 8d77e2ef

merged