Rename GPU_TIMING to EVENT_TIMING.
Init images with zero by default.
Image class returns only pointer to host data:

- update test cases to reflect this
- no need to initialize images to zero anymore
Image class returns only pointer to host data:

- update target code generation to return a pointer to the host data
Image class returns only pointer to host data:

- update runtime code to return a pointer to the host data
Documentation will be in the website repository.