add Dhgeqz and Dlapy3

[soypat]

Part of #1651.

Dhgeqz's eigenvalues do not seem to be a solution for the GNEP using input or resulting H and T matrices as far as I can tell. I'm guessing I need direct input from Dggbal? I've run comparisons of eigenvalues with reference and I'm getting identical up to tol=1e-14 results.

This is how I tried to test the solution no GNEP:

        // Compute right and left hand sides of
	//   H*x = lambda*T*x
	// Where lambda are the eigenvalues computed by Dhgeqz.
	lhs := blas64.Vector{N: n, Data: make([]float64, n), Inc: 1}
	rhs := blas64.Vector{N: n, Data: make([]float64, n), Inc: 1}
	x := blas64.Vector{
		N:    n,
		Data: randomSlice(n, rnd),
		Inc:  1,
	}
	blas64.Gemv(blas.NoTrans, 1, hCopy, x, 0, lhs)
	for i := 0; i < n; i++ {
		if alphar[i] == 0 && alphai[i] == 0 && beta[i] == 0 {
			continue // singular EV.
		}
		lambdar := alphar[i]
		lambdai := alphai[i]
		if beta[i] != 0 {
			lambdai /= beta[i]
			lambdar /= beta[i]
		}
		if lambdai != 0 {
			continue // Imag EV.
		}

		blas64.Gemv(blas.NoTrans, lambdar, tCopy, x, 0, rhs)
		if !floats.EqualApprox(lhs.Data, rhs.Data, tol) {
			t.Error("H*x != lambda*T*x", i, complex(lambdar, 0), lhs.Data, rhs.Data)
		}
       }

[codecov-commenter]

Codecov Report

Patch coverage: 78.48% and project coverage change: +0.01% 🎉

Comparison is base (93223b4) 74.00% compared to head (494271a) 74.02%.
Report is 12 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1897      +/-   ##
==========================================
+ Coverage   74.00%   74.02%   +0.01%     
==========================================
  Files         526      530       +4     
  Lines       63210    63850     +640     
==========================================
+ Hits        46778    47263     +485     
- Misses      13854    13995     +141     
- Partials     2578     2592      +14     

Files Changed	Coverage Δ
lapack/gonum/dhgeqz.go	78.44% <78.44%> (ø)
lapack/gonum/dlapy3.go	100.00% <100.00%> (ø)

... and 3 files with indirect coverage changes

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