hexify multipolygons

[knaaptime]

#125 is resolved by first exploding the input, then handling via split-apply-combine if the resulting unary_union is a multipolygon itself

[codecov-io]

Codecov Report

Merging #124 (399ea20) into master (080fca1) will increase coverage by 4.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #124      +/-   ##
==========================================
+ Coverage   78.30%   82.32%   +4.02%     
==========================================
  Files          15       15              
  Lines         742      758      +16     
==========================================
+ Hits          581      624      +43     
+ Misses        161      134      -27     

Impacted Files	Coverage Δ
tobler/tests/test_utils.py	100.00% <100.00%> (ø)
tobler/util/util.py	89.33% <100.00%> (+2.03%)	⬆️
tobler/area_weighted/area_interpolate.py	78.92% <0.00%> (+11.15%)	⬆️

---

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 080fca1...399ea20. Read the comment docs.

[martinfleis]

We do expensive source.unary_union here and then again in _to_hex or in the loop below. In larger dataframes, that can cause a significant slowdown. My sense is that you spend most of the time actually doing unary union than anything else in this function, so we surely don't want to do it twice.

[knaaptime]

good point, thanks. I added a new PR that switches to a try/except which should cut down one unary_union call when it's not necessary, unless there's yet a more efficient way you can think of?

[darribas]

For an approximate but faster solution, we could offer an option to simply cover the bounding box (or its alpha shape). That'd avoid the unary union and, in some cases, it should be the same.

Otherwise, I'd agree w/ @martinfleis: do the unary union once and re-use it if else