slight improvement for MatrixLocalScope
adjust variable scoping: only previously declared variables allowed
See #6.

Add a 'before' property of type EObject to MatrixLocalScope,
adjust constructor and factory. Filter the context contents
to only use those before 'before'.

In the scope provider, the scoping must begin on variable reference
level, i.e with the referencing variable as 'context'. Then use own
dispatch method overloads to go up the hierarchy, always using con-
tainer as new 'context' and current context as new 'before'.