Fixed the coding standards.
Issue #2829099 by GeduR: Add listClass public method to MigrateSourceList.
Issue #2654222 by das-peter: PHP7 Uniform Variable Syntax.
Issue #2552641 by T-lo, joachim: add a MigrateList class for CSV source.
Issue #2635792 by nedjo, hanoii: SQLSTATE[22007]: Invalid datetime format: 1292 Truncated incorrect DOUBLE when saving highwater.
Issue #2217909 by Placinta, bdragon: sourceMigration doesn t work if source migration map uses more than 1 destination key.