added in checkpoint options and output timestep interval

[leorosie]

• Adds the ability to write a checkpoint file at a specified time frequency
• Adds tests for checkpoint

[codecov-commenter]

Codecov Report

Merging #207 (1757966) into master (72ff281) will decrease coverage by 0.58%. The diff coverage is n/a.

:exclamation: Current head 1757966 differs from pull request most recent head 7232eda. Consider uploading reports for the commit 7232eda to get more accurate results

@@            Coverage Diff             @@
##           master     #207      +/-   ##
==========================================
- Coverage   49.78%   49.19%   -0.59%     
==========================================
  Files           4        4              
  Lines         691      683       -8     
==========================================
- Hits          344      336       -8     
  Misses        347      347              

Impacted Files	Coverage Δ
demo/richards/richards_driver.c	100.00% <ø> (ø)
demo/th/th_driver.c	100.00% <ø> (ø)

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