Fingerprint specification of versions.

[jbrezmorf]

• Add methods to add (and remove) obligatory paths that a version must have.
• Generalise version detection from 'flow123d' to any key (specific to version).
• Copy this version fingerprint from previous version.
• Remove assert for map type of the root node.

Using this general fingerprint mechanism, we can better detect version of the input file and also
distinguish YAML files that are not in particular format.