Add handling of PROJECT_DEV_VERSION in CMakelists.txt.

[nichtsfrei]

What:

Add handling of PROJECT_DEV_VERSION in CMakelists.txt.

Why:

PROJECT_DEV_VERSION is used within CMakeLists.txt to set the the version to a dev version, which needs to be set to 0 on actual release and 1 on dev versions.

if (PROJECT_DEV_VERSION)
  set (PROJECT_VERSION_SUFFIX "~dev${PROJECT_DEV_VERSION}")
endif (PROJECT_DEV_VERSION)

How:

Create a CMakeLists.txt and ru pontos release, afterwards verify the commit changes.

Checklist:

• Tests
• CHANGELOG Entry
• Documentation

[codecov]

Codecov Report

Merging #32 into master will increase coverage by 0.28%.
The diff coverage is 98.03%.

@@            Coverage Diff             @@
##           master      #32      +/-   ##
==========================================
+ Coverage   92.06%   92.34%   +0.28%     
==========================================
  Files          26       26              
  Lines        1033     1071      +38     
==========================================
+ Hits          951      989      +38     
  Misses         82       82              

Impacted Files	Coverage Δ
pontos/version/version.py	71.32% <50.00%> (+0.21%)	⬆️
pontos/release/release.py	92.46% <100.00%> (+0.05%)	⬆️
pontos/version/cmake_version.py	96.33% <100.00%> (+0.98%)	⬆️
tests/version/test_cmake_version.py	100.00% <100.00%> (ø)

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