Updated to enable use of multithreading pool when adding transforms
Slight optimization of changing stos transform scale.
Added **kwargs to transform declaration to be consistent with base class declaration
Add rbf transform that uses scipy implementation.  Does not scale to thousands of points.
Removed unneccessary copy operation
Fixed a bug where stos file dimensions were negative
Cache distance images in the temporary directory
Made flag to extrapolate points outside a transform's bounding box an option on assemble. This is needed for Pyre's auto-registration to work properly.
Minor optimization of mosaic loading
Added more restrictive add function to volume transforms to catch errors
Fixed typo
Raise a clear error if assemble is not able to load a tile.  This helps worker threads report the problem clearly to the caller.
Made use of cached distance images more robust
Merge branch 'Assemble'