Declare operator?() as 'const'.

None of operator+(), operator-(), operator*() or operator/() modify
their object.  These methods are thus naturally declared as 'const'.
Implement first cut at upwind-mobility weighting for transport

Specifically,

  - Introduce a new class, UpwindSelector<Scalar> which provides a
    selection service (method select()) to pick out a quantity across
    an interface in the upwind direction.

    Method select() is implemented only for the case of no
    counter-current flow (i.e., no effects of gravity and/or capillary
    pressure).  The selector is affected through applying an explicit
    sparse matrix to each quantity.  This may be inefficient, but only
    measurements will tell.

  - Split function fluxFunc<ADB>() into two components,
    phaseMobility<ADB>() and fluxFunc<ADB>().  The former computes
    phase mobilities (per cell) for water and oil in a subset of the
    grid cells and the latter computes the flux function (for water,
    i.e., the first phase) based on a (2-) vector of phase
    mobilities--either per cell or per internal interface.

  - Compute explicit phase mobilities (per cell) in the non-linear
    loop before employing the upwind selector to pick out proper
    upwind mobilities per (internal) interface to define the quantity
    'fw_face' which was previously computed by an arithmetic average.