Add support for object sections

Sections are an optimization mechanism that allows the partitioning of
data members of a persistent class into groups that can be separately
loaded and/or updated.

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+This example shows how to use object sections to implement lazy-loading
+and change-updating of a subset of data members in a persistent class.
+
+The example consists of the following files:
+
+person.hxx
+  Header file defining the 'person' persistent class. It contains a number of
+  data members some of which are made to belong to a section.
+
+person-odb.hxx
+person-odb.ixx
+person-odb.cxx
+person.sql
+  The first three files contain the database support code and the last file
+  contains the database schema for the person.hxx header.
+
+  These files are generated by the ODB compiler from person.hxx using the
+  following command line:
+
+  odb -d <database> --generate-schema person.hxx
+
+  Where <database> stands for the database system we are using, for example,
+  'mysql'.
+
+database.hxx
+  Contains the create_database() function which instantiates the concrete
+  database class corresponding to the database system we are using.
+
+driver.cxx
+  Driver for the example. It includes the person.hxx and person-odb.hxx
+  headers to gain access to the persistent classes and their database support
+  code. It also includes database.hxx for the create_database() function
+  declaration.
+
+  In main() the driver first calls create_database() to obtain the database
+  instance. It then persists a 'person' object, loads it back, including
+  the section, and prints the contents of its members. Finally, the driver
+  shows how to update the state of the section data members in various ways,
+  then re-loads and prints the object to verify that the changes have been
+  made persistent.
+
+To compile and link the example manually from the command line we can use
+the following commands (using  MySQL as an example; replace 'c++' with your
+C++ compiler name):
+
+c++ -c person-odb.cxx
+c++ -DDATABASE_MYSQL -c driver.cxx
+c++ -o driver driver.o person-odb.o -lodb-mysql -lodb
+
+To run the example we may first need to create the database schema (for some
+database systems, such as SQLite, the schema is embedded into the generated
+code which makes this step unnecessary). Using MySQL as an example, this
+can be achieved with the following command:
+
+mysql --user=odb_test --database=odb_test < person.sql
+
+Here we use 'odb_test' as the database login and also 'odb_test' as the
+database name.
+
+Once the database schema is ready, we can run the example (using MySQL as
+the database):
+
+./driver --user odb_test --database odb_test