Change summary:
1. DenseMatrixOpsExp: Moving purgeSymbols within matrixMult so that
there are fewer symbols to purge. Waiting for the whole autotuner to
finish would mean that there are many many symbols to purge, which is
taking a long time

2. DenseMatrixOpsImpl: Removed hardcoded numParamsPerLevel and using the
field within Tunable. Also some transpose stuff has leaked into this
change; more on transpose in the coming changes.

3. Tunable: Added some helpful comments to explain the seemingly
arbitrary constants