DR_Voelund_Hill2: Bivariate dose-response function under Voelund model
In gilles-guillot/MDR: Test Loewe additivity with Model Deviation Ratio

Compute bivariate dose-response function when each mixture compoent follows a 2-parameter Hill dose-response function and the Voelund model holds.

1	DR_Voelund_Hill2(d1, d2, a1, b1, a2, b2, lambda1, lambda2)

`d1`	dose of first compound: a single numeric entry
`d2`	dose of second compound: a single numeric entry
`a1`	parameter a in Hill-2 DR fuction 1/(1+(a/x)**b) for first compound
`b1`	parameter b in Hill-2 DR fuction 1/(1+(a/x)**b) for first compound
`a2`	parameter a in Hill-2 DR fuction 1/(1+(a/x)**b) for second compound
`b2`	parameter b Hill-2 DR fuction in 1/(1+(a/x)**b) for second compound
`lambda1`	parameter Voelund model
`lambda2`	parameter Voelund model