Change the formatting of prior distributions

[renecotyfanboy]

Change the structure of the input dict of prior. This will greatly simplify the way models are constructed and make it consistent across the various functions we use.

E.G. switch from

{ 
    "tbabs_1":
        {
            "N_H" : dist
        }
}

TO

{ 
    "tbabs_1_N_H": dist
}

[renecotyfanboy]

Partially landed with v0.1.1