added new model parameter API.

I have started to implement basic WCS functionality in `specutils` and worked with the new `Model` class from `astropy.modeling`. This showed me that the functionality in `Model` is quite good, however the API is a bit "clunky". I've worked a lot with the `sqlalchemy` - an object relational mapper. They represent their Tables (or table rows) as classes and have implemented a simple, but powerful API. Each individual column has a name, a type (and other properties like Foreignkey). This is very similar in my mind to a Model which has Parameters which have properties (e.g. fixed). Attached my suggestion of what I feel would make `Model` much easier to work with (mostly when writing your own).

What are people's thoughts.