Add ability to group datasets into collections of related datasets

Issue Name

Add ability to group datasets into collections of related datasets

Summary

It could be useful to have a data structure or class that sits between proposal ID and dataset that would allow datasets to be grouped into collections of related datasets. In our use case, we are using the proposal object for unique user proposals that would ultimately translate to a project or publication and the dataset object for sets of data resulting from a single process on a given machine. In many cases we expect to have an experiment in which many related datasets would be generated, for example Optical Electron Spectrometry, XPS, and TEM, where it would be useful to group the dataset objects together to form a distinct collection of data within the proposal / project.

Start by tracing the existing proposal and dataset objects to understand how they are represented and related. Define how an intermediate collection would fit between them and how related datasets would be grouped within a proposal. Done means the project has an agreed data structure or class that supports these collections.