Initial conditions and default parameters handling

[YingboMa]

We need

    initial_conditions::Any # [x => 1.0, y => 2.0, z => 2y]
    # observed: [y ~ 2x]
    # [x => 1.0, y => 2*(2x)]
    default_parameters::Any

fields in various systems, so that a user can write

eqs(params) = ...
model = (name, iv, (structural_params, params)) -> ODESystem(eqs(structural_params), pins=[F, H, K...], default_params = ...)

to instantiate a new model. Default parameters are going to be merged with params using merge(default, params) so that params is preferred.