Python: split API in reader/writer modules

babeltrace package's modules are now:

  common:
    class CTFStringEncoding
    class ByteOrder
    class CTFTypeId
    class CTFScope
  reader: everything related to reading
  writer: CTF writer API

Backward compatibility is ensured by importing the
appropriate modules into the package itself.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Python: examples: import explicit BT modules

The best practice in Python is generally to import
explicit modules rather than importing all its exposed
members.

This also makes the examples hide the fact that
reader/writer members are exposed directly by the package,
whereas this is only a backward compatibility layer now.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Python: document writer.EnumerationMapping

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Python: document writer.Clock

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Python: document writer.FieldDeclaration

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Python: add top-level class writer.IntegerBase

Documenting, and using in general, top-level classes
is easier than nested classes in Python.
babeltrace.writer.FieldDeclaration.IntegerBase still
exists for backward compatibility.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Python: document writer.IntegerFieldDeclaration

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Pytnon: document writer.EnumerationFieldDeclaration

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Python: document writer.FloatingPointFieldDeclaration

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Python: document writer.StructureFieldDeclaration

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Python: document writer.VariantFieldDeclaration

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Python: document writer.ArrayFieldDeclaration

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Python: document writer.SequenceFieldDeclaration

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Python: document writer.StringFieldDeclaration

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Python: document writer.Field

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Python: document writer.IntegerField

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Python: document writer.EnumerationField

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Python: document writer.FloatingPointField

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Python: document writer.StructureField

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Python: document writer.VariantField

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>