Minor fixes for FaceCellsProxy::operator().

Fixes are:
 - Made the method const.
 - Corrected argument name cell_index -> face_index.
Use GridHelpers in implementation of vertexNeighbours().
Whitespace: tabs to spaces.
Rename vertexNeighbours() -> cellNeighboursAcrossVertices().
Replace M_PI with boost usage.
Add skeleton of AnisotropicEikonal class and test.
Added simple test, work in progress.
Work in progress on AnisotropicEikonal.
Solver logic finished.

Methods isClose(), computeFromTri() and computeFromLine() are still dummies.
Initial versions of compute functions done.

Still has bugs.
Work in progress.
Fix bug in distance derivative function.
Fix bug in test.

Also add (inactive) second test.
Follow renaming of function.