Standardized names for methods, added missing methods

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Description

Extending select() to support indexed column access. Select is the non-modifying equivalent of retainColumns().
Rename select() to selectColumns() for consistency
Add rejectColumns() to support the non-modifying (i.e., return a new table) exclusion of columns. This is the equivalent of removeColumns().
Add methods to Relation:

• ColumnType[] typeArray()
• List<ColumnType<?> types()
• containsColumn(String columnName);
  Deprecated method ColumnType[] columnTypes(), because methods with similar names return Lists not Arrays.
  Added method append(Row), and deprecated addRow(Row) so that append() is always the name whether the argument is a table, slice, or row.

Testing

Did you add a unit test?
Yes, and updated the old ones.