move ".cite" string to docstring & return docstring

[rabitt]

the current implementation of cite is (e.g. from dali.py)

def cite():
    """Print the reference"""

    cite_data = """
    ===========  MLA ===========
    Meseguer-Brocal, Gabriel, et al.
    "DALI: a large Dataset of synchronized Audio, LyrIcs and notes, automatically created using teacher-student machine
    learning paradigm."
    In Proceedings of the 19th International Society for Music Information Retrieval Conference (ISMIR). 2018.

    ========== Bibtex ==========
    @inproceedings{Meseguer-Brocal_2018,
    Title = {DALI: a large Dataset of synchronized Audio, LyrIcs and notes, automatically created using teacher-student
     machine learning paradigm.},
    Author = {Meseguer-Brocal, Gabriel and Cohen-Hadria, Alice and Peeters, Geoffroy},
    Booktitle = {19th International Society for Music Information Retrieval Conference},
    Editor = {ISMIR}, Month = {September},
    Year = {2018}}
    """
    print(cite_data)

Our docs for this method are not very helpful, and it would be great if the reference was itself part of the docs.

We could instead move the citation directly to the docstring:

def cite():
    """
    ===========  MLA ===========
    Meseguer-Brocal, Gabriel, et al.
    "DALI: a large Dataset of synchronized Audio, LyrIcs and notes, automatically created using teacher-student machine
    learning paradigm."
    In Proceedings of the 19th International Society for Music Information Retrieval Conference (ISMIR). 2018.

    ========== Bibtex ==========
    @inproceedings{Meseguer-Brocal_2018,
    Title = {DALI: a large Dataset of synchronized Audio, LyrIcs and notes, automatically created using teacher-student
     machine learning paradigm.},
    Author = {Meseguer-Brocal, Gabriel and Cohen-Hadria, Alice and Peeters, Geoffroy},
    Booktitle = {19th International Society for Music Information Retrieval Conference},
    Editor = {ISMIR}, Month = {September},
    Year = {2018}}
    """
    print(cite.__doc__)

Alternatively, we could define CITE_DATA as a global in bibtex format, which would get us closer to #227