Add select method for bool tensors

[poshcoe]

Feature description

Expose select and select_assign tensor operations for boolean tensors.

At the moment the select and select_assign methods are within the Numeric trait, however the functionality is also useful for boolean tensors (and I don't think there are any numeric requirements to its implementation)

Feature motivation

The select method is very handy for float and int tensors. Bool tensors have been left out!

Suggest a Solution

My high level suggestion is moving these methods to the BasicOps trait.