Introduce devito.Zero to replace sympy.Zero in the FD layer

[FabioLuporini]

this way we avoid annoying try-excepts such as

       try:
             theta = model.theta
         except AttributeError:
             theta = 0

or

if theta != 0:
    <expr>

[mloubout]

It's not that simple

• first case has nothing do with FD layer, just the case where the model doesn't have a theta attribute only option is try/catch or getattr
  The second case has so many possibilities tracking them all would be quite a pain