[WIP} Added a WIP array.mean() implementation.

Related to issue #828 and some others.
Is that the behavior of numpy?  Does anyone read this have a 32 bit-build they can test?

I have a slight preference in keeping the bit-depth on this high, as accumulators tend to roll over more often than you would think.  Is there a significant performance trade-off to emulating the higher bit-depth on older machines?