implemented rule to constraint assignments only to signals

[rutefig]

Implemented the rule to only allow assignments (<-- and <==) to signals and did a simple test to check if we throw the error when we are doing that to a wg var

[leolara]

Perfect!! The only thing perhaps the message, because = will allow assignment to wgvars. So perhaps we can say that you cannot use <-- but can use = or something like that