add support for citations attribute

[sahiljhawar]

Solves #362
Uses a Mixin base class with citation attribute to be inherited by other lightcurve classes.

Example usage:

kn = SVDLightCurveModel(model="Bu2019lm",sample_times=[0,0.5],svd_path="path/to/svd",local_only=True,filters=["2massh"])
grb  = GRBLightCurveModel(sample_times=[0,0.5])

combined = GenericCombineLightCurveModel(models=[kn,grb],sample_times=[0,0.5])

combined.citation
>>> [{'Bu2019lm': ['https://arxiv.org/abs/2002.11355',
    'https://arxiv.org/abs/1906.04205']},
    {'TrPi2018': ['https://arxiv.org/abs/1909.11691']}]

kn.citation
>>>{'Bu2019lm': ['https://arxiv.org/abs/2002.11355',
   'https://arxiv.org/abs/1906.04205']}

grb.citation
>>>{'TrPi2018': ['https://arxiv.org/abs/1909.11691']}

Few citations are missing

[mcoughlin]

I think this looks good, let's just use a better name than Mixin.

[mcoughlin]

LGTM