hotfix: import error

[fletchapin]

Pull request recommendations:

• Name your pull request {your development type}: {short description}. Ex: Feature: flocculator validation
• Link to any relevant issue in the PR description. Ex: Resolves Remove all hard-coded constants from utility files #12, adds flocculator validation
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Resolves #322 by removing string from the beginning of unit_definitions.txt. Also fixes the SyntaxWarning by changing is to == with string comparisons

[codecov]

Codecov Report

Merging #324 (194442d) into master (cc40815) will not change coverage.
The diff coverage is 60.00%.

@@           Coverage Diff           @@
##           master     #324   +/-   ##
=======================================
  Coverage   89.56%   89.56%           
=======================================
  Files          29       29           
  Lines        3047     3047           
=======================================
  Hits         2729     2729           
  Misses        318      318           

Impacted Files	Coverage Δ
aguaclara/core/constants.py	100.00% <ø> (ø)
aguaclara/core/pipes.py	98.48% <ø> (ø)
aguaclara/research/peristaltic_pump.py	100.00% <ø> (ø)
aguaclara/design/floc.py	90.24% <33.33%> (ø)
aguaclara/core/cache.py	94.28% <100.00%> (ø)
aguaclara/core/units.py	81.81% <100.00%> (ø)

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