Add AbstractTrees Interface

[JBlaschke]

It exposes an AbstractTrees.jl interface via the HwlocTreeNode. Hwloc is already a tree, but it's not easy to use from Julia, so this makes things like tree traversal, and iteration, from Julia much easier.

This a follow-up for: #71

[codecov-commenter]

Codecov Report

Attention: 55 lines in your changes are missing coverage. Please review.

Comparison is base (772b6d3) 28.36% compared to head (6d4c0e2) 33.67%.
Report is 2 commits behind head on master.

Files	Patch %	Lines
ext/HwlocTrees.jl	16.66%	40 Missing ⚠️
src/highlevel_api.jl	85.07%	10 Missing ⚠️
src/lowlevel_api.jl	90.19%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #72      +/-   ##
==========================================
+ Coverage   28.36%   33.67%   +5.31%     
==========================================
  Files           4        5       +1     
  Lines         846      962     +116     
==========================================
+ Hits          240      324      +84     
- Misses        606      638      +32     

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[carstenbauer]

Two comments from my side:

1. Sounds like a pretty good use case for a Pkg Extension: the tree functionality would only become automatically available when one loads both Hwloc and AbstractTrees in the same session. OTOH, AbstractTrees is very lightweight, e.g. doesn't have any dependencies, so it might be fine as is.

2. Needs documentation, at least an example of how to use it for the README.

[JBlaschke]

All done @carstenbauer -- now using Pkg Extensions. And updated README. Should I merge?

[carstenbauer]

LGTM but since the README here contains the changes from PR #71 already I guess you should wait with merging until that one has landed.

BTW, I found a typo in the README: "eccident" should be "accident".

[JBlaschke]

LGTM but since the README here contains the changes from PR #71 already I guess you should wait with merging until that one has landed.

Yep. I'll wait for your approval on #71 and #72 and then merge both in that order

BTW, I found a typo in the README: "eccident" should be "accident".

Fixed it! Thanks.

[carstenbauer]

I will merge this together with #71 in about 6 hours if there are no objections.