Improve csv

[casenave]

scalars and splits in csv feature very long lines, which beats the purpose of having human readable file.
How to improve ? (car change to yml or other file format)

[xroynard]

We should switch to .json or .yaml format, it is more suitable to store dictionnaries

see #14 for retrocompatibility

[xroynard]

for splits in ProblemDefinition, it should be solved by #183