More groundwork in the compiler for building the AST when parsing
stencils.

- Updated lexer to allow for the '$' to indicate a StencilStatement.
- Added StencilAccessNode and StencilStatementNode
- Added handleStencilStatement
- Updated signature of handleStencilAccess
- Updated sample program for cartesian.
Merge branch 'cartesian' of https://github.com/sintefmath/equelle into cartesian