Improve conversion from Chains to InferenceData

[sethaxen]

This PR simplifies and streamlines conversion from Chains to InferenceData. It also fixes the handling of observed_data and constant_data following the schema, adds new functions for creating Dataset, removes mcmcchains_summary from attributes (it breaks serialization with netcdf), and handles concat more safely.

[codecov]

Codecov Report

Merging #24 into master will increase coverage by 0.68%.
The diff coverage is 97.29%.

@@            Coverage Diff             @@
##           master      #24      +/-   ##
==========================================
+ Coverage   86.66%   87.35%   +0.68%     
==========================================
  Files           6        6              
  Lines         225      253      +28     
==========================================
+ Hits          195      221      +26     
- Misses         30       32       +2

Impacted Files	Coverage Δ
src/utils.jl	51.28% <0%> (-8.18%)	⬇️
src/data.jl	90.19% <100%> (+5.82%)	⬆️
src/mcmcchains.jl	100% <100%> (+1.09%)	⬆️
src/dataset.jl	100% <100%> (+2.38%)	⬆️

---

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 ec1e7e4...31ea17a. Read the comment docs.