Properly close heatmap figure after use

[TimJentzsch]

Relevant issue: Closes #151

Description:

The pyplot figures to create the heatmap weren't closed properly after use. This resulted in them cluttering up the memory.

Checklist:

• [x] Code Quality
• [x] Pep-8
• [ ] Tests (if applicable)
• [x] Success Criteria Met
• [ ] Inline Documentation
• [ ] Wiki Documentation (if applicable)