Element-wise sum and product

[Ababwa]

Use case for vectors:
Product for calculating volume.
Sum for getting taxicab distance.

Not sure what uses cases there might be for matrices, but seems apt to include them for completeness.

[bitshifter]

Happy to add product and sum for vectors, naming wise probably something like product_elements and sum_elements to differentiate from the existing product and sum implementations and to match other element wise methods like max_element. I wouldn't add these to matrices unless there's a concrete use case.

[Ababwa]

How about element_product and element_sum?

[bitshifter]

Sure, I like sum_elements but product_elements sounds weird.

[bitshifter]

Merged in #467