Create a new DataLoader instance.

[gagnonanthony]

As part of the transition toward classes rather than functions, create a DataLoader object to load data from data frames and graph networks with a consistent structure with constant attributes.

This should be able to do the following:

• Save the data into any desired format (tabulated form or graph network).
• Compute the graph network layout if not available.
• Visualize as a graph network.
• Easy addition of new data for an existing subject or new subjects