Fix crash in finalisation of the standalone post processing

For some reason, when compiling with cmake, ths standalone job crashes in the finalisation of a correlation plot.

The changes the type of fit that root is trying and so should no longer crash.

Interestingly, when compiled outside athena, or with the athena release, but with a standalone makefile, this runs fine.

This code changes only affects a standalone executable that we run in our nightly tests - it is not executed as part of any standard athena test, and is not executed, and has no influence on any of the required CI tests.