Reorder children in tree

[colganwi]

Added reorder_children method to CassiopeiaTree. This is useful for reordering clades prior to plotting a tree.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.02% :tada:

Comparison is base (43e6b2e) 78.80% compared to head (1ade18d) 78.82%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #217 +/- ## ========================================== + Coverage 78.80% 78.82% +0.02% ========================================== Files 88 88 Lines 7774 7782 +8 ========================================== + Hits 6126 6134 +8 Misses 1648 1648 ``` | [Files Changed](https://app.codecov.io/gh/YosefLab/Cassiopeia/pull/217?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=YosefLab) | Coverage Δ | | |---|---|---| | [cassiopeia/data/CassiopeiaTree.py](https://app.codecov.io/gh/YosefLab/Cassiopeia/pull/217?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=YosefLab#diff-Y2Fzc2lvcGVpYS9kYXRhL0Nhc3Npb3BlaWFUcmVlLnB5) | `91.57% <100.00%> (+0.10%)` | :arrow_up: |

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[colganwi]

I think a separate function that organizes the entire tree with respect to a meta data item would make the most sense. But I'm not sure we want to include this in Cassiopeia since it requires significant customization. What's the order of the items? How to handle missing data? How to order children with impure subtrees? How to organize with respect to multiple metadata columns? Probably better to let each user write their own function that calls reorder_children.

[mattjones315]

All good points. I'll make an issue for providing an example in the existing plotting tutorial (just an enhancement). If it's alright, I'll assign it to you for when you have time to provide this example.

Thanks for discussion, and I'll go ahead and merge this in.