Add specifications for maximum and minimum

[kgryte]

This PR

• resolves RFC: add support for computing the element-wise minimum and maximum #667 by adding specifications for computing the element-wise minimum and maximum.
• as in min and max, complex number support is left unspecified. Input data types should be real-valued data types.
• as the APIs are straightforward and consistent across array libraries, the changes proposed in this PR do not have any open questions requiring resolution.

[kgryte]

As (1) the additions of minimum and maximum have previously been discussed in workgroup meetings and #667, (2) all considered array libraries already implement these element-wise APIs, and (3) no objects have been raised here, will go ahead and merge. Any further changes can be addressed in follow-up PRs.