Fix(steps): Typing steps are bypassed

Useless steps where complex objects were built are not shown. I create
a typing function which is called when compute raise
NotImplementedError.

mapytex/calculus/API/expression.py

@@ -10,7 +10,7 @@
 Expression
 
 """
-from ..core import Tree, compute
+from ..core import Tree, compute, typing
 from .renders import renders
 
 class Expression(object):
@@ -104,9 +104,11 @@ class Expression(object):
         <Exp: 2 + 12>
         """
         try:
-            t = self._tree.apply_on_last_level(compute)
+            t = self._tree.apply_on_last_level(compute, typing)
         except AttributeError:
             return self
+        except NotImplementedError:
+            return self
         else:
             e = Expression(t, ancestor=self)
             return e.simplify()