Export function does not export AD and RD

[DelinteNicolas]

elikopy/elikopy/core.py

Line 1734 in c41753f

if dti:

The export function does not currently export the AD and RD metrics from the DTI folder.

[DelinteNicolas]

Additionally, it would be nice to add an argument allowing a custom folder to be exported, in addition to the 'pre-selected' ones