SchemaImpl (Sparqlify SPARQL->SQL rewriter

Constructors
Constructor and Description
`SchemaImpl()` An empty schema without any columns.
`SchemaImpl(List<String> names, Map<String,TypeToken> nameToType)`
`SchemaImpl(List<String> names, Map<String,TypeToken> nameToType, Set<String> nullableNames)`

Methods
Modifier and Type	Method and Description
`static SchemaImpl`	`create(List<String> names, Map<String,TypeToken> nameToType)`
`Schema`	`createSubSchema(List<String> columnNames)`
`boolean`	`equals(Object obj)`
`int`	`getColumnCount()`
`String`	`getColumnName(int index)`
`List<String>`	`getColumnNames()`
`TypeToken`	`getColumnType(int index)`
`TypeToken`	`getColumnType(String name)`
`Map<String,TypeToken>`	`getTypeMap()`
`int`	`hashCode()`
`boolean`	`isNullable(String columnName)`
`String`	`toString()`
`static void`	`validateNames(List<String> names)`

Methods inherited from class java.lang.Object
clone, finalize, getClass, notify, notifyAll, wait, wait, wait

SchemaImpl
```
public SchemaImpl()
```
An empty schema without any columns. FIXME If zero-columns schema cause troubles, we will change the behaviour of this ctor to create a single dummy column

public SchemaImpl(List<String> names,
          Map<String,TypeToken> nameToType)

public SchemaImpl(List<String> names,
          Map<String,TypeToken> nameToType,
          Set<String> nullableNames)

public static void validateNames(List<String> names)

public static SchemaImpl create(List<String> names,
                Map<String,TypeToken> nameToType)

public Schema createSubSchema(List<String> columnNames)

Class SchemaImpl