Introduced new level-3 front-end layer.

Details:
- Added new _front() functions for each level-3 operation. This is done
  so that the choosing of the control tree (and *only* the choosing of
  the control tree) happens in what was previously the "front end"
  (e.g. bli_gemm()). That control tree is then passed into the _front()
  function, which then performs up-front tasks such as parameter
  checking.