3Delight auto NSI export format based on file extension

[gkocov]

Generally describe what this PR will do, and why it is needed

• DelightRender node NSI scene description export format is based on file extension - .nsi for binary and .nsia for ASCII

Breaking changes

• .nsi file extension export is now binary, while previously it was ASCII (not sure if this applicable since it's not a API/ABI change, but wanted to list it just in case)

Checklist

• I have read the contribution guidelines.
• I have updated the documentation, if applicable.
• I have tested my change(s) in the test suite, and added new test cases where necessary.
• My code follows the Gaffer project's prevailing coding style and conventions.

[johnhaddon]

Thanks Goran! Again, I've rebased to fix the conflict in Changes.md (they crop up a lot, because most PRs edit that file). Merging...