modified collision with exact kernel, closes #158

[glyg]

Plus some import reorganisation

[coveralls]

Coverage decreased (-0.2%) to 80.038% when pulling 30ee632 on glyg:master into ffd668d on DamCB:master.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@ffd668d). Click here to learn what that means.
The diff coverage is 79.16%.

@@            Coverage Diff            @@
##             master     #160   +/-   ##
=========================================
  Coverage          ?   80.17%           
=========================================
  Files             ?       83           
  Lines             ?     4746           
  Branches          ?        0           
=========================================
  Hits              ?     3805           
  Misses            ?      941           
  Partials          ?        0

Impacted Files	Coverage Δ
tyssue/behaviors/monolayer/actions.py	82.22% <100%> (ø)
tyssue/behaviors/__init__.py	100% <100%> (ø)
tyssue/behaviors/sheet/basic_events.py	82.85% <50%> (ø)
tyssue/behaviors/sheet/actions.py	76.04% <92.3%> (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 ffd668d...23f2a6b. Read the comment docs.