Make naming consistent for precondtions and postconditions.

[bdice]

Description

Resolves #216.

Types of Changes

• Documentation update
• Bug fix
• New feature
• Breaking change¹

¹The change breaks (or has the potential to break) existing functionality.

Checklist:

• I am familiar with the Contributing Guidelines.
• I agree with the terms of the Contributor Agreement.
• My name is on the list of contributors.
• My code follows the code style guideline of this project.
• The changes introduced by this pull request are covered by existing or newly introduced tests.

If necessary:

• I have updated the API documentation as part of the package doc-strings.
• I have created a separate pull request to update the framework documentation on signac-docs and linked it here.
• I have updated the changelog.

[codecov]

Codecov Report

Merging #433 (a1a5f65) into master (03fe709) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #433   +/-   ##
=======================================
  Coverage   70.44%   70.44%           
=======================================
  Files          29       29           
  Lines        2994     2994           
  Branches      553      553           
=======================================
  Hits         2109     2109           
  Misses        737      737           
  Partials      148      148           

Impacted Files	Coverage Δ
flow/project.py	73.64% <100.00%> (ø)

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