fix Quantity resize in Series.append

Created by: stefco

described in issue #758, https://github.com/gwpy/gwpy/issues/758

When trying to append to a Series instance that already has _xindex initialized with an astropy.units.Quantity value, an error is thrown due to the fact that Quantity only owns a view to its own data and cannot therefore be resized.

Fix this by trying to copy the underlying numpy array in quantity.value, resizing that copy, and then creating a new Quantity instance with the resized array and the original quantity.unit.