Export to png file in SA and PI histogram

[rwmcintosh]

Fixes #2595 Sensitivity Analysis plot: Export to PNG not implemented
Fixes #2594 PI Histogram of Residuals plot: Export to PNG not implemented

Description

Implementing the export for other chart types

Type of change

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• 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 not work as expected)
• This change requires documentation changes (link at least one user or developer documentation issue):
• Algorithm update - updates algorithm documentation/questions/answers etc.
• Other (please describe):

How Has This Been Tested?

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• Integration tests
• Unit tests
• Manual tests
• No tests required

Reviewer checklist

Mark everything that needs to be checked before merging the PR.

• Check if the code is well documented
• Check if the behavior is what is expected
• Check if the code is well tested
• Check if the code is readable and well formatted
• Additional checks (document below if any)
• Check if documentation update issue(s) are created if the option This change requires a documentation update above is selected

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