Merge remote-tracking branch 'origin/feature/7394_polref_idf'
Merge remote-tracking branch 'origin/bugfix/7199_vesuvio_fitting_diffs'
Merge remote-tracking branch 'origin/feature/7144_new_v_conv_spectrum_axis'
Merge remote-tracking branch 'origin/feature/7397_reflectometry_md_splitting'
Re #6168. Copy over code to track performance from main repository.

It's not ideal to fork things in this way, but I don't want to have to
check out the mantid repository in order to run this on the system tests.
Merge branch 'feature/6168_performance_timing' into develop