taco expressionless mode

[fredrikbk]

We should extend the taco tool to support a workflow where it is not given an expression, but instead computes using only input kernels.

Example:

taco -t=a,B,c -f=B:ds -i=B:mat.tns -i=c:vec.tns -read-source=spmv.c

The -t option specifies the tensors in the kernel. The list is ordered and the first tensor is the result.

Alternatively:

taco -f=B:ds -i=B:mat.tns -i=c:vec.tns -read-source=spmv.c:a,B,c