new simplified API to fill sparse matrices (the old functions are

deprecated). Basically there are now only 2 functions to set a
coefficient:
1) mat.coeffRef(row,col) = value;
2) mat.insert(row,col) = value;
coeffRef has no limitation, insert assumes the coeff has not already
been set, and raises an assert otherwise.
In addition I added a much lower level, but more efficient filling
mechanism for
internal use only.

Eigen/src/Sparse/CompressedStorage.h

@@ -155,7 +155,7 @@ class CompressedStorage
     /** Like at(), but the search is performed in the range [start,end) */
     inline Scalar atInRange(size_t start, size_t end, int key, Scalar defaultValue = Scalar(0)) const
     {
-      if (start==end)
+      if (start>=end)
         return Scalar(0);
       else if (end>start && key==m_indices[end-1])
         return m_values[end-1];