XCore: fix modelDirectory
fix typo
add common supertype for unary expressions
add common super type for binary expressions
minor reordering in xcore model
XCore: make a bunch of classes abstract
XCore: types implement equals(), hashCode()
matrix entries must inherit from Number
interpretation: do not do null check in equality

Null value should not occur. If it does, fail intentionally.
fix scoping from matrix init

A fix to commit d026a1d192: when scoping out of a matrix init
expression, getScope() was called instead opf scopeVaraible(),
leading to the whole Code to be searched, not a surrounding func
declaration.
remove intentionally incorrect example code now that it is detected
Another fix to commit d026a1d192: scope argument must be nullable

When scoping for a proc or func, the 'before' argument is null.
As Guice throws an exception on null arguments unless they are
annotated with @Nullable, add such an annotation.

The mentioned annotation resides in jsr305-1.3.9.jar.
add quick dependency note to README
interpreter: inject ResourceSet instead of using explicit implementation
some minor changes