Make config files more readable

[hammannr]

In binference, the definition of a parameter in the config was distributed over the entire file and it was easy to lose the overview.
I'd suggest using a structure like the following to increase the readability of parameter initialization in alea:

parameter_definition = {
    wimp_mass: {
        nominal_value: 50,
        fixed: True,  # or use floating instead?
    },
    livetime_0: {
        nominal_value: 1.,
        fixed: True,
    },
    livetime_1: {
        nominal_value: 1.,
        fixed: True,
    },
    signal_rate_multiplier: {
        nominal_value: 1.,
        type: "rate",  # or fix all of this internally in the likelihood i.e. inspect from parameter name what the parameter does.
        fixed: False,
    },
    er_rate_multiplier: {
        nominal_value: 1.,
        type: "rate",
        uncertainty: .2,
        relative_uncertainty: True,
        fixed: False,
    },
    par1: {
        nominal_value: 0,
        type: "shape",
        uncertainty: $some_expression_to_define_a_constraint,  # we could allow any scipy and np expression e.g.
        relative_uncertainty: False,
        fixed: False,
    },
}

[hammannr]

After discussion with @kdund : Probably more meaningful to define a boolean fittable instead of fixed.