Characteristic Equations and 1D/0D Fluid Coupling

This branch includes characteristic equations, which describe wave propagation phenomena and are solved using a discontinuous nodal solution method. These equations are used in concert with the 1D approximation of the Navier-Stokes equations allowing for branching in vascular trees. 

Lumped parameter (0D) CellML coupled models at 1D boundaries may be described using algebraic and/or time integrated DAEs. Coupling between the 0D models and the 1D Navier-Stokes/Characteristic equations is facilitated by bridging routines in the Navier-Stokes routines that translate the returned CellML pressure values in terms of the Characteristics and 1D Navier-Stokes variables.

The 1D/0D coupling portion of this pull request falls under Tracker item 3374 – Multiscale modelling for fluid problems.

@Soroosh to review