make EclDefaultMaterial three-phase API conformant
fix headercheck
add a tabulated piecewise linear two-phase material law

this is intended to be used for ECLIPSE saturation curves, but the
code is more generic. (Be aware that using cubic splines for the
interpolation between sampling points is probably a better choice in
most cases.)
Capillary pressure laws: enforce that parameter objects are finalized before they are used

this only has an effect if compiled in debug mode, i.e., if the NDEBUG
symbol is not defined.