ENH: refactoring of the sensitivity classes

Before the commit, the sensitivity classes were receiving references of
the (incompressible) primal and adjoint variables.  However, if
additional physics was added (energy equation, multiphase, etc), the
infrastructure wasn't convenient for accommodating (new terms in the FI
and E-SI formulations, new terms in the sensitivity map, etc).

Now, the sensitivity classes receive a reference to an
incompressibleAdjointSolver and receive the terms for the FI and
sensitivity maps through there. The latter is still WIP.

Modified adjointSimple to incorporate these changes as well.