Added (a lots of) comments to code; unused methods cleanup

TASMAN-core/src/main/java/it/inaf/ia2/tsm/Column.java

TASMAN-core/src/main/java/it/inaf/ia2/tsm/Key.java

TASMAN-core/src/main/java/it/inaf/ia2/tsm/KeyHolder.java

TASMAN-core/src/main/java/it/inaf/ia2/tsm/Schema.java

TASMAN-core/src/main/java/it/inaf/ia2/tsm/TSMUtil.java

TASMAN-core/src/main/java/it/inaf/ia2/tsm/Table.java

TASMAN-core/src/main/java/it/inaf/ia2/tsm/TapSchema.java

...N-core/src/main/java/it/inaf/ia2/tsm/TapSchemaMender.java

...core/src/main/java/it/inaf/ia2/tsm/TapSchemaSettings.java

...-core/src/main/java/it/inaf/ia2/tsm/UpdateOperations.java

TASMAN-core/src/main/java/it/inaf/ia2/tsm/WrongDataType.java

...AN-core/src/main/java/it/inaf/ia2/tsm/datalayer/ADQL.java

.../src/main/java/it/inaf/ia2/tsm/datalayer/Credentials.java

...ore/src/main/java/it/inaf/ia2/tsm/datalayer/DBBroker.java

...re/src/main/java/it/inaf/ia2/tsm/datalayer/DBWrapper.java