Add new function: allparents

[cserteGT3]

For my application a function was missing: getting all the parents of a cell. This PR adds this function (similar to the other all...() functions).
Edit: I did not write a test for it.

[codecov-io]

Codecov Report

Merging #21 into master will decrease coverage by 1.32%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #21      +/-   ##
==========================================
- Coverage   80.41%   79.09%   -1.33%     
==========================================
  Files           5        5              
  Lines          97      110      +13     
==========================================
+ Hits           78       87       +9     
- Misses         19       23       +4

Impacted Files	Coverage Δ
src/RegionTrees.jl	100% <ø> (ø)	⬆️
src/cell.jl	76.66% <100%> (+2.66%)	⬆️
src/hyperrectangle.jl	90% <0%> (-10%)	⬇️
src/twosarray.jl	72.22% <0%> (-4.25%)	⬇️

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[rdeits]

Looks good, thanks! Could you please add a test for the new function?

[cserteGT3]

Sorry for the late reply (had to finish the semester), I added some tests for the new function.
I'm not experienced in writing tests, so please review it, if they meet the goal.

[rdeits]

Thank you, this looks great!