New numerical steepest descend based on Gauss Hermite qudrature
Remove some unused code
Improve path precomposition
Fix wrong path parametrization in new nsd code
Document the untransformed Gauss Hermite quadrature rule.
Fix severe bug in NSD implementation.

Note that the bug only occurs for D >= 3 and
not for D = 1 or D = 2.
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
Merge pull request #89 from raoulbq/smolyak_improve

Smolyak improve