marginalized gaussian mixture model

[OriolAbril]

File: https://github.com/pymc-devs/pymc-examples/blob/main/examples/mixture_models/marginalized_gaussian_mixture_model.ipynb
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[chiral-carbon]

taking this issue up

[chiral-carbon]

@OriolAbril I was looking at this and #102 and as asked in issue #32, should we try to combine this notebook with gaussian mixture model notebook?