Fix a TimestepValue offset indexing bug

[davidanthoff]

No description provided.

[codecov]

Codecov Report

Merging #857 (0544445) into master (b74d386) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #857   +/-   ##
=======================================
  Coverage   84.30%   84.30%           
=======================================
  Files          40       40           
  Lines        3792     3792           
=======================================
  Hits         3197     3197           
  Misses        595      595           

Flag	Coverage Δ
unittests	84.30% <100.00%> (ø)

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

Impacted Files	Coverage Δ
src/core/time_arrays.jl	93.00% <100.00%> (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 b74d386...0544445. Read the comment docs.

[lrennels]

Added tests