Make last equation of a node potentially not ending with a semi column

Otherwise the following code:

```
-- count the number of top between two tick
node counting (tick:bool; top:bool)
returns (o: bool);
	var v: int;
	let o = if tick then v else 0 -> pre o + v;
		v = if top then 1 else 0
	tel;
```

was involving the following error:

```
Syntax error at <line 1: -- count the number of top between two tick >
```

src/parser.mly

@@ -276,6 +276,7 @@ equations:
   | /* empty */ { [] }
   | equation SEMICOL equations
       {  $1 :: $3 }
+  | equation OPTIONAL_SEMICOL { [$1] }
 ;
 
 equation: