CenterAwareComputerOps that can operate without OutOfBoundsFactory

`CenterAwareComputerOps` require an `OutOfBounds` strategy to access pixels in neighborhoods that do not completely cover the image area (/volume, I will stick to the 2D case here). There are, however, filters that could compute exact values for those neighborhoods by adapting their computation scheme.

For example, `MeanFilterOp` could compute the mean value based on the available image pixels, effectively ignoring the out of bounds strategy. I don't know how those cases are handled in other libraries/frameworks/etc and if we should stick to the current behavior of `CenterAwareComputerOps` when no `OOBF` is provided by the user (i.e. assuming an `OOBF` that is used for the computations).

What do you think about that @dietzc, @bnorthan, anyone?