New Model constructor breaks examples

[cwrowley]

The new constructor in Model (introduced in #14) breaks some examples, e.g., those in ode.py. In particular, any subclass of Model that doesn't re-define an __init__ method will break, unless an rhs method is provided to the constructor. (And Model subclasses usually include an rhs method, so it would be silly to provide this to the constructor.)

There are also some bugs in this constructor: for instance, all 3 if statements check whether adjoint_rhs is set.

I am also not a fan of this whole setattr way of doing this. I'm sure we can come up with something better.