Fix in Mooney-Rivlin density: Use proper 4th term for Ciarlet

In Ciarlet the last factor is left unspecified. I could not follow the choice used in this implementation. I guess it was intended to minimize the energy under a hydrostatic stress F = t*I at the unit matrix, i.e., for t=1. A simple calculation shows that (independent on the dimension)

d = 2a + 4b + 2c

has to hold in this case. Values in the test were adjusted.