Implement "joining" multiple parameters in @defcomposite

[corakingdon]

#657
and I've also implemented the various set_param! methods described in #524

[codecov]

Codecov Report

Merging #687 into master will increase coverage by 0.60%.
The diff coverage is 93.04%.

@@            Coverage Diff             @@
##           master     #687      +/-   ##
==========================================
+ Coverage   79.20%   79.80%   +0.60%     
==========================================
  Files          39       38       -1     
  Lines        2851     2837      -14     
==========================================
+ Hits         2258     2264       +6     
+ Misses        593      573      -20     

Flag	Coverage Δ
#unittests	79.80% <93.04%> (+0.60%)	⬆️

Impacted Files	Coverage Δ
src/core/instances.jl	78.30% <0.00%> (ø)
src/core/model.jl	81.81% <ø> (ø)
src/core/paths.jl	79.31% <ø> (-3.05%)	⬇️
src/core/show.jl	9.75% <ø> (+0.38%)	⬆️
src/core/defs.jl	82.12% <81.81%> (+1.23%)	⬆️
src/core/defcomposite.jl	89.10% <92.85%> (+6.83%)	⬆️
src/core/connections.jl	87.03% <96.15%> (-0.47%)	⬇️
src/core/build.jl	95.89% <100.00%> (+1.01%)	⬆️
src/core/references.jl	53.33% <100.00%> (+14.87%)	⬆️
src/core/types/core.jl	91.66% <100.00%> (+1.19%)	⬆️
... and 8 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b052bf2...d7244a0. Read the comment docs.