this patch converts to code to use the convenience functions instead
of the math toolboxes whereever possible. the main advantage is that
Opm::foo(x) will work regardless of the type of `x`, but it also
reduces visual clutter.
also, constant Evaluations are now directly created by assigning
Scalars, which removes further visual noise.
while I hope it improves the readability of the code,
functionality-wise this patch should not change anything.
0f6540bdad