Added IECorePython::CastToIntrusivePtr.

This can be used as a return value policy when binding a C++ function returning a raw pointer which must be cast to an intrusive_ptr to allow python to share ownership. Previously it was necessary to manually wrap every single such function to return an intrusive_ptr, and then to bind that.
Fixed FileSequence API to use raw pointers where appropriate.
Fixed IECoreGL::Shader API to use raw pointers where appropriate.
Fixed Renderer::Procedural API to use raw pointers where appropriate.
Fixed Parameterised API to use raw pointers where appropriate.
Fixed Display API to use raw pointers where appropriate.
Fixed Camera API to use raw pointers where appropriate.
Fixed CoordinateSystem API to use raw pointers where appropriate.
Fixed Group API to use raw pointers where appropriate.
Fixed IECoreGL::Texture API to use raw pointers where appropriate.