Make a separate class for CompositeModel, simplifying Model, and supporting add, subtract, multiply, and divide for Models 

This change introduces a `CompositeModel` class, a subclass of `Model` that holds two `Model`s and a binary operator for them.   
This is meant to replace and expand the current mechanism for adding two `Model`s.  The PR passes tests, but is put forth mainly 
for discussion. The main features of the PR are:

   1.  the `Model` class itself simplified.  Many of the internal methods in `Model` had conditional code for whether the `Model` had 
        *components* or not, often with the distinction between the two cases (is a composite model, is not a composite model) being 
        hard to follow.  With this PR, a basic `Model` wraps a model function, but cannot hold sub-components.  Instead, adding or 
        multiplying 2 models creates a `CompositeModel`.   By itself, this simplification of `Model` seems like a good idea to me.

   2. While the current code supports adding `Model`s, this PR also supports subtraction, multiplication, and division of `Model`s.   
       Admittedly, the need for these is less urgent and obvious than adding `Model`s, but it is useful to have.   

   3. Because the code to support a binary operation is minimal, and relies of Python builtins.  That is, the 
       code to support division is:

```Python
     def __div__(self, other):   # in class Model: how to divide Models
        return CompositeModel(self, other, '/')
```

and

```Python
    def eval(self, params=None, **kwargs): # in class CompositeModel
        lval = self.left.eval(params=params, **kwargs)
        rval = self.right.eval(params=params, **kwargs)
        # ....
        elif self.op == '/':    return lval / rval
```

Normal precedence rules are enforced by Python, and parentheses can be used as needed, so that one could do

```Python
    full_model = Model1 * (Model_A + Model_B/Model_C)
```

With this approach, it would be trivial to add support for other of Python many binary operators.  For example, it 
would be possible to override one of these (perhaps `%`, '^', or '&') to signify convolution, or some other operation.
I think it's an open question whether that is a good idea or not.

In short -- non-trivial features are added, supporting more mathematical operations of `Model`s, with additional 
supported operators very easy to add.   Furthermore, the `Model` class is simpler, and the code in model.py is 
only a few lines longer than the current version. The PR currently passes all tests, but no new tests or docs have 
been added.  This is mostly a request for comments and suggestions on how to improve this idea.