added support for user options in sx renderer
added support for multiple repeated shading grids
Removed conflicting parameter type warning for coshaders

This was generating reams of spurious warnings in large coshader networks
Merge pull request #240 from davidsminor/sxRendererFixes

Sx renderer fixes
Refactored IECore::StandardRadialLensModel and tidied the comments.
Simplified the arithmetic and refactored it so that it matches that of the paper it is implemented from.
Merge pull request #241 from goddardl/LensModelRefactor

Refactored IECore::StandardRadialLensModel and tidied the comments.
Updating changes and bumped minor version
Fixes crash when merging CortexPrimitives in Houdini 13.

Added merge constructor and fixed signature of stashed() for H13
Merge pull request #242 from andrewkaufman/mergeConstructor

Fixes crash when merging CortexPrimitives in Houdini 13.
Fixed a bug where the EXRImageReader would interpret Deep EXRs as valid.
Merge pull request #244 from goddardl/EXRImageReaderBug

Fixed a bug where the EXRImageReader would interpret Deep EXRs as valid.