Better MKL

This PR fixes a few problems to make the MKL interfaces more robust, particularly when working with very large matrices. All users should upgrade.

Per NEP 29, this release also drops support for Python 3.6 and NumPy 1.16.

Merged PRs

• Gate large-memory tests based on available memory (#12) @mdekstrand
• Support very large matrix multiplication with MKL (#11) @mdekstrand
• Enforce MKL interface limits (#10) @mdekstrand
• Further updates to version testing (#9) @mdekstrand
• Fix creation of large empty matrices (#8) @mdekstrand