Add implementation objects and more API functions

[sethaxen]

This PR follows the example of posteriordb-python introducing an AbstractImplementation type with concrete types StanModelImplementation and PyMC3ModelImplementation. These types will make it easier to support implementations for more frameworks in the future.

It also renames load_values to load and introduces the function path.

[codecov]

Codecov Report

Merging #8 (be0703f) into main (e12c4a7) will increase coverage by 0.74%.
The diff coverage is 96.15%.

@@            Coverage Diff             @@
##             main       #8      +/-   ##
==========================================
+ Coverage   79.61%   80.35%   +0.74%     
==========================================
  Files           7        8       +1     
  Lines         103      112       +9     
==========================================
+ Hits           82       90       +8     
- Misses         21       22       +1     

Impacted Files	Coverage Δ
src/PosteriorDB.jl	100.00% <ø> (ø)
src/model.jl	41.17% <ø> (-8.83%)	⬇️
src/implementation.jl	92.30% <92.30%> (ø)
src/dataset.jl	50.00% <100.00%> (-3.34%)	⬇️
src/posterior_database.jl	95.00% <100.00%> (+0.26%)	⬆️
src/reference_posterior.jl	87.50% <100.00%> (-1.39%)	⬇️

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