Added top-level override() function.

The function is implemented in the new functiontools package.

The function is added to the global namespace when you started Abjad.

The function can be used as a drop-in replacement for the bound 'override'
property:

    >>> staff = Staff("c'4 d'4 e'4 f'4")
    >>> override(staff).staff_symbol.color = 'blue'

    >>> f(staff)
    \new Staff \with {
            \override StaffSymbol #'color = #blue
    } {
            c'4
            d'4
            e'4
            f'4
    }

DEPRECATION: the bound 'override' property is now deprecated.
Use the new override() function instead.
Updated import statement in new override.py module.