Change mboTensorOpMatVec interface.

This function should take raw pointers to the x and y arrays to decouple from
MboVec.  In addition, it needs to take a range of rows [rmin, rmax) to make it
more consistent with the other MboTensorOp methods and to allow chunking of rows
for better cache utilization and threading.

Examples compile and run successfully.
Convert test_DenseMatrix to new mboTensorOpMatVec api.
Remove restrict keyword.

Requires more thought to make this work portably for C, C++, and different
platforms.
Convert test embedding to [rmin, rmax) api.
Convert remaining tests to [rmin, rmax) API.
Merge pull request #6 from d-meiser/fix-mat-vec-api

Fix mboTensorOpMatVec API