Make linear interpolation default for saturation and some pvt properties.

Saturation properties includes relative permeability and capillary pressure functions.

PVT properties include dead oil and dead gas tables.

The default has been to make a monotone spline from the given table values and use a fine, uniform sampling of that. With this change 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.

(Note that the commit message is a bit misleading, since it only mentions saturation properties.)