stencil and gstencil inherit from stencil_base
stencil and gstencil inherit from stencil_base

refs #4
Extracted general data and code to a base class. Problematic halo
exchange code is now defined at a single place.

kapadia/vexcl,ddemidov/vexcl,ddemidov/vexcl,Poulpy21/vexcl,kapadia/vexcl,Poulpy21/vexcl,Poulpy21/vexcl