Fix rounding of position values when converting from integer to float in _fixupPosition

[ianmcorvidae]

Fixes #572

Luckily, decimal is standard library, so this is pretty easy to fix.

The issue here is this:

>>> Decimal(1e-7)
Decimal('9.99999999999999954748111825886258685613938723690807819366455078125E-8')
>>> Decimal("1e-7")
Decimal('1E-7')

So some positions were ending up as the wrong value due to the multiplication with not quite an actual power of 10.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 61.82%. Comparing base (4d67e7f) to head (e8dfee8).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #585      +/-   ##
==========================================
+ Coverage   61.81%   61.82%   +0.01%     
==========================================
  Files          14       14              
  Lines        2941     2942       +1     
==========================================
+ Hits         1818     1819       +1     
  Misses       1123     1123              

Flag	Coverage Δ
unittests	61.82% <100.00%> (+0.01%)	⬆️

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