ANA-513: refactor request errors hqndling in connection
columns.label_references_cp should be None by default, useful for AlterColumn migration
add column.__eq__ and column.__ne__ methods
change altercolumn management of label_references_cp
in retrieve_attr_tuples function, when calculating Labels, save if they references a connection point or not
add a DiffState object and State.get_remote_diff to compute the differences between two versions of the dataset
initialize the MigrationEngine class that will automatically create a migration
ANA-513: resolving merge problems
ANA-513: review fixes
in column comparison: bugfix and compare the type of the column too
columns References should not have a title, it is meaningless
bugfix and testing the DiffState object
make it possible to instanciate a migration chain with a chain and a data_chain
complete the migration engine with the generate_chain function
add State.is_synchronised method to check that the class is synchronised with the API
add TestMigration.test_engine method
bugfix in test_migration.test_engine: should alter a copy of the class, not the class directly
ANA-513: review fixes #1
ANA-513: handle request errors in connection for delete also
ANA-516: review fixes