First implementation of the Smolyak construction
Document Smolyak quadrature scheme
Abort Smolyak construction if there are not enough high order rules
Fix errors if rule is not constructed yet
Fix a bug when reshaping factors inside the integration routines
Make transformation of Gauss Hermite QR weights more efficient

This removes also a numpy warning about a division by zero.
The division never caused issues becaue the data affected
was never used.
Clean up tensor product computations
Catch invalid Smolyak levels
Improve node construction exploiting symmetry, part 1
Improve node construction exploiting symmetry, part 2