bind: chaining binds together

[fjebaker]

Suppose we had two models, A and B. We bound A.K_1 to B.K_2 but then bound A.K_1 to A.K_2. We now will get an index error when we try to get the parameters for model B because the parameter array of A has one parameter fewer.

Possible solution:

• during bind!, check if we are binding a parameter that is already due to be bound. If so, append new bindings to them.

[phajy]

Not strictly a chaining problem but perhaps a related bug: I noticed that when, e.g., the second model was bound to the first model so they should be identical the model parameters were, in fact, different.

julia> result[1]
┌ FittingResultSlice:
│   Model: CompositeModel[XS_Jet + XS_Optxagnf]
│   . u     : [3.0, 10.0, 0.20000, 10.0, 2.1000, 0.00010000, -1.0000, 3.0, 13.0, 0.10000, 1275.0, 2.6000, 43.300, 10.990, 300.0, 3000.0, 0.98000, 2.7000]
│   . σᵤ    : [nothing]
│   . χ²    : 396.92 
└ 

julia> result[2]
┌ FittingResultSlice:
│   Model: CompositeModel[XS_Jet + XS_Optxagnf]
│   . u     : [3.0, 0.20000, 10.0, 2.1000, 0.00010000, -1.0000, -1.0000, 3.0, 13.0, 0.10000, 1275.0, 2.6000, 43.300, 10.990, 300.0, 3000.0, 0.98000, 2.7000]
│   . σᵤ    : [nothing]
│   . χ²    : NaN 
└ 

I've put this issue here as a placeholder while I investigate further (I still need to confirm it is actually a bug and not something I've done!) I will post an update here when I've dug a bit deeper...