fixed rdf lib bug

[MauriceHendrix]

Description

Motivation and Context

fixes #344

Types of changes

• [X] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Documentation checklist

• [x] I have updated all documentation in the code where necessary.
• [ ] I have checked spelling in all (new) comments and documentation.
• [x] I have added a note to RELEASE.md if relevant (new feature, breaking change, or notable bug fix).

Testing

• [x] Testing is done automatically and codecov shows test coverage
• [ ] This cannot be tested automatically

[codecov[bot]]

Codecov Report

Merging #345 (0361d66) into master (d6fe513) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #345   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            9         9           
  Lines         1543      1543           
  Branches       347       366   +19     
=========================================
  Hits          1543      1543           

Impacted Files	Coverage Δ
cellmlmanip/model.py	100.00% <100.00%> (ø)

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