Generalize numel() that counts the volume of a slice

[jfalcou]

No description provided.

[jfalcou]

Fixed with merge of #50

[jfalcou]

As of the merge of #50, shape supports slicing operations that return new shapes on which we can apply numel().
I am going to write down a couple pages of documentations for this then close this issue.

[psiha]

waiting for this :) but do consider (ct) efficiency, i.e. overhead of slicers for just, in effect, reduced fold range (than the one in numel() ;P

[jfalcou]

the API is rbr based as usual and has already be simplified. I guess I'll need some TRIVIAL/PURE massaging again.
We support complex slicing so the geenral case is not just a reduced fold range.