Updates TimeSincePrevious and Diff Primitives

[ctduffy]

fixes #501

[codecov]

Codecov Report

Merging #561 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #561      +/-   ##
==========================================
- Coverage   96.36%   96.35%   -0.01%     
==========================================
  Files         116      116              
  Lines        9262     9244      -18     
==========================================
- Hits         8925     8907      -18     
  Misses        337      337

Impacted Files	Coverage Δ
...s/tests/primitive_tests/test_transform_features.py	98.31% <100%> (ø)	⬆️
...turetools/computational_backends/pandas_backend.py	98.06% <100%> (-0.02%)	⬇️
...ols/tests/dfs_tests/test_deep_feature_synthesis.py	100% <100%> (ø)	⬆️
...retools/primitives/standard/transform_primitive.py	100% <100%> (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 c139527...f644e22. Read the comment docs.

[rwedge]

In general, the tests that used Diff / TimeSincePrevious should use GroupByTransformFeature so that we do not need to change the test answers.

[ctduffy]

fixed issues brought up by @rwedge

[ctduffy]

@rwedge fixed test cases

[rwedge]

Looks good

[ctduffy]

@kmax12 fixed the timesinceprevious units and removed the diff date example

[kmax12]

LGTM