Add dependency of equelle_parser.y and equelle_lexer.l when building el.
Started updating the compiler with support for "stencil-variables."

- Added new basicTypes StencilI, StencilJ and StencilK.
- Updated the various EquelleType::is<Numeric|Entity|...>Type function
with the new stencil-types.
- Added method EquelleType::isStencilType.
- Added stub methods handleStencilAccessStatement and
handleStencilAccess
- Updated lexer to allow for the '@' char to be used for stencil-
- Updated parser to handle stencil-access.
- Added the built-in functions StencilI() and StencilJ() to the symbol-
- Added example program (in experimental/cartesian/