black oil density: correct the gas mass fraction in saturated oil
peng-robinson test: fix it, so that it produces correct output
refactor the 2D tabulation classes

- the StaticTabulated2DFunction class and the base class
  (Tabulated2DFunction) are gone
- the DynamicTabulated2DFunction class has been renamed to
  UniformTabulated2DFunction
- a new class called UniformXTabulated2DFunction has been
  introduced. Like UniformTabulated2DFunction, it assumes uniform
  intervalls of the sampling points in X direction, but in contrast to
  UniformTabulated2DFunction, the Y locations of the sampling points
  can be set freely (as long as they are specified in increasing order
  for each x value)
- add a unit test for the two tabulation classes
black oil system: use the UniformXTabulated2DFunction class for the oil formation factor

this means that undersaturated oil is now properly supported.
Black-oil fluid system: do not explicitly include the composition in the tabulated density

It might have to be included, but it made the reservoir test in eWoms
fail...