Fix reference before assignment on utci heatplot generation

[Tom-Kingstone]

NOTE: Depends on

Issues addressed by this PR

Closes #246

replaced use of base64 with save_path when asking to save the plots

Test files

Test using the current beta test script

Changelog

Additional comments

ensure you don't count the test done off this PR as a beta artefact test

[Felix-Mallinder]

Tested with LadybugTools_Toolkit_TestScript.gh and all test run successfully return True . Happy to approve these changes.

[Tom-Kingstone]

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[jamesramsden-bh]

This seems a very sensible change