Moving partial PDFs to self._dependent_variables?

**Describe the solution you'd like**
Currently, the partial pair distribution functions are stored within the PDF class in their own class attibute - `self.partial_pdfs`

We should consider whether or not the partial PDFs are to be stored alongside the total PDF in the `self._dependent_variables` or if they should remain as they are. While this could help usability and code quality, it could also be entirely unnecessary. I am soliciting opinions on if this is a good idea or not.

Start by reading the PDF class and tracing how self.partial_pdfs and the total PDF are stored in self._dependent_variables. Compare the usability and code-quality implications of both arrangements; done requires a documented decision on whether partial PDFs should move, followed by any agreed implementation.

Written by the indexing model from the issue text.