Potentially change how build handles default values

[corakingdon]

The current behavior for setting default values is that the build function calls _set_defaults! on the model definition before running the model. The _set_defaults! function creates external parameters for any unconnected parameters that have default values. This means that the model definition is modified by the run function. Specifically, there will be extra parameters in the external_params list after running that weren't there before. This could cause confusion, in particular about when a user can use set_param! versus update_param! to change a parameter's value.

This PR changes the behavior of the build function such that the _set_defaults! function is called on the deepcopy of the model definition that gets stored in the created model instance, instead of on the original model definition. I think this makes for more intuitive behavior of the parameters. See the added test file here for the expected behavior.

I've marked this as draft because we should definitely discuss this.

Note: if we go with this, we'll need to check if there are cases in the models' code that use set_param/update_param at the wrong time

[codecov]

Codecov Report

Merging #702 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #702   +/-   ##
=======================================
  Coverage   80.05%   80.05%           
=======================================
  Files          38       38           
  Lines        2858     2858           
=======================================
  Hits         2288     2288           
  Misses        570      570           

Flag	Coverage Δ
#unittests	80.05% <100.00%> (ø)

Impacted Files	Coverage Δ
src/core/build.jl	100.00% <100.00%> (ø)
src/core/defcomp.jl	91.19% <0.00%> (ø)
src/core/connections.jl	87.03% <0.00%> (ø)

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