External data: sha512 or md5

[SimonRit]

We currently have both sha512 and md5 files for external data.
See for example https://github.com/RTKConsortium/RTK/tree/master/test/Input/Bioscan.
Should we keep both? Any opinion @LucasGandel?

[SimonRit]

@LucasGandel, do you remember why we have both sha512 and md5?

[LucasGandel]

Not sure why, I think we always use sha512, but ITK had both at some point too. Now they moved to .cid : InsightSoftwareConsortium/ITK#3760, maybe we should do the same

[SimonRit]

I think md5 can be removed, see #660. I have opened an issue for .cid, #661.