[WIP] Optimized Advection-Diffusion-Reaction example

This branch is to address issue #44 
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[x] assemble diffusion-reaction version from existing optimized models
[ ] verify implementation on our standard heterogeneous manufactured solution
[ ] add advection term from VOF model
[ ] verify on 1D analytical solution 
[ ] add doc strings and doc tests to ADR.pyx
[ ] add doxygen docs to ADR.h