Fix orthotropic material

[WeiyiVirtonomy]

See issue #680

The major changes are:

1. Fix CalculateAllLambda()
2. Fix CalculateAllMu()
3. Fix the wrong parentheses and the integer division 1/2 (!) in PK2 stress computation
4. Use the maximum bulk modulus in three principle directions $\max (\frac{1}{3}(\sum_j \lambda_{ij} + 2\mu_i))$ instead of maximum E and nu to compute K0 and c0, as the relation between K and E, nu no longer holds for anisotropic material

TODO:

1. Add parameter check
2. Implement Cauchy stress
3. Add unit test