Make linear interpolation default for saturation properties.

This includes relative permeability and capillary pressure functions.
The default has been to make a monotone spline from the given table
values and use a fine, uniform sampling of that. Now the default
is to use the tables as-is. It is still possible to use the spline
approach. For example in the class BlackoilPropertiesFromDeck one
may pass nonzero values for the 'pvt_tab_size' and 'sat_tab_size'
parameters, corresponding to how fine the spline will be sampled.
Merge pull request #497 from atgeirr/no-spline-default

Make linear interpolation default for saturation and some pvt properties.