Allow to add annotations to Cfg elements

[jenetics]

It should be possible to add annotations to all Cfg elements like Cfg.Rule, Cfg.Terminal or Cfg.NonTerminal. This annotations can than be used to influence the program generation from an CFG. Such an annotation could be a weight for a given element, which is than be used to influence the selection probability during program generation.

Example
A BNF representation might look like this:

<expr> ::= <num> | <var> @weight=4 | '(' <expr> <op> <expr> ')' @weight=5
<op>   ::= + | - | * | /
<var>  ::= x | y
<num>  ::= 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9

In this example, the <num> rule would have a selection probability of 1/10 and the <var> rule of 4/10.

With such annotation it will be possible to generate programs with the desired properties much easier.