Add duecredit

[rmarkello]

A lot of the code in this repository builds on a significant amount of previous work and publications. Currently, references to those works are located in the doc-strings of relevant functions/classes (under the References heading). However, we could instead integrate duecredit, a tool that helps users know to whom and how to assign credit when using code.

Thankfully, the duecredit page has a brief walk-through for integration. It would be great to integrate it into pyls and convert all the references currently located in doc-strings into duecredit-style citations.

As an example, pyls.types.BehavioralPLS currently has the following reference in its doc-string:

class BehavioralPLS(BasePLS):
    """
    Runs "behavioral" PLS

    Parameters
    ----------
    ...
    
    References
    ----------
    .. [1] McIntosh, A. R., Bookstein, F. L., Haxby, J. V., & Grady, C. L.
       (1996). Spatial pattern analysis of functional brain images using
       partial least squares. Neuroimage, 3(3), 143-157.
    """

This could be converted to a duecredit-style citation:

@due.dcite(BibTeX("""
    @article{mcintosh1996spatial,
    author={McIntosh, AR and Bookstein, FL and Haxby, James V and Grady, CL},
    title={Spatial pattern analysis of functional brain images using partial least squares},
    year={1996},
    journal={NeuroImage},
    volume={3},
    number={3},
    pages={143--157}}
    """),
    description='First application of PLS to functional neuroimaging data.')
class BehavioralPLS(BasePLS):
    """
    Runs "behavioral" PLS

    Parameters
    ----------
    ...
    """

Then, users would be able to follow the instructions for running their code with duecredit enabled to get a functioning BibTeX file!

It would be best to follow the steps in the duecredit guide for copying over the stub.py to the primary directory of pyls and then importing (from .due import due, Doi, BibTeX) as required. The rest would be filling out the BibTeX-style decorators, like in the example above.