Fix some docstrings

Raise not implemented errors for cost methods
Add the concept of a serializable fitting result

It basically just contains the shapes, a copy of the parameters
and the initial image. It needs comment strings and discussing.
Update the documentation for serializable fitters
Fix various bugs in fitting images

We need the parameters to be a list within the fitting
framework. I also updated the tests to be passing.
A bit of a dirty commit

1. Change shapes to a property, remove the as_points option
2. Update the compute error to reflect the above change
3. Change shapes to parameters on NonParametric
4. Remove HDF5 Init
Refactor multilevel to contain multiple serialized

Now the serialized multilevel acts more like the normal multilevel
and just contains a list of serialized fitting results
Add a couple of tests for serializable
Merge pull request #445 from patricksnape/serialise_fit_results

Serialize fit results