MAINT: Improve computation of scaled companion matrices.

The previous method used for hermite and hermite_e polynomials suffered
from double overflow for polynomials of large degree. Those numbers were
later scaled down by equally large numbers, but the result was NaN. The
wanted values are now computed in such a way that overflow in some
entries is replaced by underflow in others. The resulting zeros are a
negligible perturbation of the companion matrix.
MAINT: Improve computation of gauss weights for hermite and hermite_e.

The new computations use normalized polynomials. Those polynomials are
less subject to overflow and allow the determination of the weights and
roots up to about degree 350, at which point the weights underflow.

Two private functions are added for computing the values of the
normalized polynomials:

1. _normed_hermite_n
2. _normed_hermite_e_n
Merge pull request #5154 from charris/improve-gauss-hermite

Improve gauss hermite