No version check when opening a file

[dylseven]

PyPlanning version is reported in the XML planning file but not loading in memory nor checked when opening it. This can lead to incompatibilities when opening planning from a newer version, with confusing exceptions (attributes that doesn't exist...).

Fix : include the planning file's version in the loaded object, check it and deny files from newest versions than the current one with a clear message, to force the user to update.