added ConservativeSparseSparseProduct.h
overloaded ConservativeSparseSparseProduct to speed up matrix-matrix multiplication.
overloaded ConservativeSparseSparseProduct to speed up matrix-matrix multiplication.
added some comment about sort.
use correct types of SparseMatrices.
added collapseJacs method.
include vector to make compile on proprietary systems.
avoid the multiplication with zero matrix entries.
avoid multiplication with empty matrices.
if one of the matrices involved does not contain nonZeros, we can leave the product
routine.

Conflicts:
	opm/autodiff/ConservativeSparseSparseProduct.h
Merge branch 'PR/performance-avoid-zero-matrix-entries' of ssh://github.com/dr-robertk/opm-autodiff into PR/performance-avoid-zero-matrix-entries

Conflicts:
	opm/autodiff/ConservativeSparseSparseProduct.h
cleanup
remove typename.
move Index query into if.
Add Robert Kloefkorn's fast sparse product implementation.
Use fastSparseProduct(), do not use hijacked Eigen header.
Use fastSparseProduct() in place of Eigen's product.
Redo commit 6f55c862 after rebasing.
Merge pull request #1 from atgeirr/performance-mods

Performance mods from atgeirr, great job.
[cleanup] I revert the sort call to QuickSort again since it's faster on my machine.