* Refactored GafferImage::Filter to have a better interface whilst removing the need for the construct() method.
* Changed the Sampler to use the new Filter interface. Filters now have to be passed into the Sampler's constructor.
* Fixed bugs in the Sampler that:
	- Caused incorrect results with negative sample areas.
	- Samples required to compute a filtered sample were being clamped to the sample area even when the resulting sample was inside.
* Modified the Reformat node to use the new Filter and sampler interface.
* Updated bindings for the Sampler and Filter classes.
* Added filtering support for the ImageTransform node.
Added a shared look-up table for the filter class.
 * Chaged the filters to use floats after running some tests and finding out that the use of doubles made no difference to the results.
 * Fixed a few bugs with the sampler and refactored after doing some profiling.