Refactor handling of three digit groups

[poneill]

Hi,

I've tried to refactor the handling of three digit groups a bit, and I've added a few tests to cover cases that were previously handled incorrectly. This fixes #27 (and possibly others?)

Cheers,
Pat.

[codecov]

Codecov Report

Merging #34 into master will increase coverage by 1.51%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #34      +/-   ##
==========================================
+ Coverage    96.8%   98.31%   +1.51%     
==========================================
  Files           2        2              
  Lines         125      119       -6     
==========================================
- Hits          121      117       -4     
+ Misses          4        2       -2

Impacted Files	Coverage Δ
word2number/w2n.py	97.26% <100%> (+2.13%)	⬆️
unit_testing.py	100% <100%> (ø)	⬆️

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