Add time_units enumeration

[benjeffery]

Fixes #1772

[codecov]

Codecov Report

Merging #1832 (9e9251f) into main (631ed78) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

❗ Current head 9e9251f differs from pull request most recent head 3840429. Consider uploading reports for the commit 3840429 to get more accurate results

@@            Coverage Diff             @@
##             main    #1832      +/-   ##
==========================================
- Coverage   93.43%   93.39%   -0.04%     
==========================================
  Files          27       27              
  Lines       24804    24820      +16     
  Branches     1094     1094              
==========================================
+ Hits        23175    23181       +6     
- Misses       1594     1604      +10     
  Partials       35       35              

Flag	Coverage Δ
c-tests	92.12% <ø> (-0.08%)	⬇️
lwt-tests	89.14% <ø> (ø)
python-c-tests	94.53% <100.00%> (+<0.01%)	⬆️
python-tests	98.75% <100.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
c/tskit/core.h	100.00% <ø> (ø)
c/tskit/tables.c	90.17% <ø> (-0.13%)	⬇️
python/_tskitmodule.c	91.68% <100.00%> (+<0.01%)	⬆️
python/tskit/__init__.py	100.00% <100.00%> (ø)
python/tskit/drawing.py	99.42% <100.00%> (+<0.01%)	⬆️
python/tskit/trees.py	97.84% <100.00%> (ø)
python/tskit/metadata.py	98.84% <0.00%> (-0.01%)	⬇️
python/tskit/exceptions.py	100.00% <0.00%> (ø)

---

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 631ed78...3840429. Read the comment docs.

[jeromekelleher]

Looks good, but maybe we should change the DEFAULT to UNKNOWN?