Change NewDense signature and behaviour

This is an API breaking change.

NewDense now panics if len(mat) != r*c, unless mat == nil. When mat is
nil a new, correctly sized slice is allocated.

mat64/matrix.go

@@ -289,7 +289,7 @@ func Inverse(a Matrix) *Dense {
 	for i := 0; i < m*m; i += m + 1 {
 		d[i] = 1
 	}
-	eye, _ := NewDense(m, m, d)
+	eye := NewDense(m, m, d)
 	return Solve(a, eye)
 }