split ReadOnlyColumnCollection from writeable methods

split ReadOnlyColumnCollection from writeable methods

The :class:`.ColumnCollection` class hierarchy has been refactored to allow
column names such as ``add``, ``remove``, ``update``, ``extend``, and
``clear`` to be used without conflicts. :class:`.ColumnCollection` is now
an abstract base class, with mutation operations moved to
:class:`.WriteableColumnCollection` and :class:`.DedupeColumnCollection`
subclasses. The :class:`.ReadOnlyColumnCollection` exposed as attributes
such as :attr:`.Table.c` no longer includes mutation methods that raised
:class:`.NotImplementedError`, allowing these common column names to be
accessed naturally, e.g. ``table.c.add``, ``table.c.remove``,
``table.c.update``, etc.

Change-Id: I22da8314fe7c451003e948d774040d86901bbca4