name clash of package variables and module names

I just realized that the name clash of package variables and module names in cern.cpymad has some unpleasant consequences:

```python
>>> import cern.cpymad.madx as m
>>> m
<class 'cern.cpymad.madx.Madx'>
```

I also suspect that this name clash is the reason for various import problems I had in the past (c.f. 459897b). Note, that the behaviour changes, if the submodule is first imported after the package variable is defined (it will be overwritten, then).

The only way to fix is, is to either rename the `.madx` module or the variable `madx` inside the `cern.cpymad` package. Both of these are not fully backward compatible.