Obtaining viscosity by doing a linear interpolation of the inverse of B*mu instead of mu directly.  

With this PR, when we do PVT calculation of viscosity, we do the linear interpolation of the inverse of B*mu. It turned out to agree with the Eclipse better. The effects are huge when the the PVT tables are coarse. So far, it improved the the results of SPE1 as the following images show. It does not show obvious effects on SPE3 and SPE9. It improves the results of some of our reduced Norne cases a lot. It did not show any bad effects so far. 

Before this PR
![spe1_gor](https://cloud.githubusercontent.com/assets/6847941/4356165/4acf0e40-4256-11e4-8355-07a87034f4a8.png)
After this PR
![spe1_gor_after](https://cloud.githubusercontent.com/assets/6847941/4356167/4e281ffa-4256-11e4-8d1f-f28142af07aa.png)


With our simplified 2D cases, the results are very good with dead oil and dead gas, or with either live oil or wet gas. The results with both live oil and gas remain to be improved. The reason is not known and some new knowledge with be helpful.