Add tests for field accessors; add docs

[codecov[bot]]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (52f6265) 80.14% compared to head (ee46eed) 71.25%.

:exclamation: Current head ee46eed differs from pull request most recent head 35d9b2c. Consider uploading reports for the commit 35d9b2c to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## mainline #530 +/- ## ============================================ - Coverage 80.14% 71.25% -8.90% ============================================ Files 70 69 -1 Lines 6055 5465 -590 Branches 510 0 -510 ============================================ - Hits 4853 3894 -959 - Misses 1202 1571 +369 ``` | [Files](https://app.codecov.io/gh/dmlc/treelite/pull/530?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dmlc) | Coverage Δ | | |---|---|---| | [python/treelite/model.py](https://app.codecov.io/gh/dmlc/treelite/pull/530?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dmlc#diff-cHl0aG9uL3RyZWVsaXRlL21vZGVsLnB5) | `84.73% <100.00%> (+8.93%)` | :arrow_up: | | [python/treelite/model\_builder.py](https://app.codecov.io/gh/dmlc/treelite/pull/530?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dmlc#diff-cHl0aG9uL3RyZWVsaXRlL21vZGVsX2J1aWxkZXIucHk=) | `95.00% <ø> (+2.50%)` | :arrow_up: | | [src/field\_accessor.cc](https://app.codecov.io/gh/dmlc/treelite/pull/530?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dmlc#diff-c3JjL2ZpZWxkX2FjY2Vzc29yLmNj) | `79.31% <50.00%> (+79.31%)` | :arrow_up: | ... and [51 files with indirect coverage changes](https://app.codecov.io/gh/dmlc/treelite/pull/530/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dmlc)

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.