Support computations using float16

[mtbarta]

We use sgemm to run matmuls for every query.

We can cut this in half by supporting float16, which MKL/OpenBLAS support.

Requirements:

• We need to be able to toggle this behavior. We should expect some perf loss from the loss in precision.
• We need to decide when float16 computation is allowed. e.g. Are we casting our stored embeddings to float16 instead of float32? Where does that happen?