BUG: fix Graph.to_W

[martinfleis]

I've noticed that when ids are not ordered, they get ordered within groupby which messes up the conversion to W. This seems to fix it.

[codecov]

Codecov Report

Merging #551 (683b3ce) into geographs (bd228c7) will increase coverage by 0.1%.
The diff coverage is 100.0%.

@@             Coverage Diff             @@
##           geographs    #551     +/-   ##
===========================================
+ Coverage       81.1%   81.1%   +0.1%     
===========================================
  Files            127     127             
  Lines          14549   14560     +11     
===========================================
+ Hits           11794   11814     +20     
+ Misses          2755    2746      -9     

Files Changed	Coverage Δ
libpysal/graph/base.py	98.2% <100.0%> (+<0.1%)	⬆️
libpysal/graph/tests/test_base.py	100.0% <100.0%> (ø)

... and 1 file with indirect coverage changes

[jGaboardi]

LGTM. Let's merge after Windows passes.

[jGaboardi]

(as a side note, I've never even heard of pandas.factorize(). You really do learn something new every day!)