Issue118 misaligned blocks in gpu codes

[tkphd]

Details about this contribution:

This PR resolves the following issues:

• Properly aligns caching tiles in CUDA and OpenCL diffusion code, fixing bugs that produced partial diffusion for most grid sizes (issues OpenCL bugs #96, generic tiled convolution excludes right boundary #112, and misaligned blocks in gpu codes #118)
• Drastically reduces the number of host-to-device data transfers in OpenAcc diffusion code, dropping runtimes from 45× to 4× that of CUDA (issue OpenACC is slow #49)
• Converts static to dynamic allocation of the shared tile cache for CUDA and OpenCL diffusion codes, making the code much more flexible (issue dynamic tile size #113)
• Renames variables from (col,row) to (x,y), reflecting the physical nature of the work (issue physical variable names #114)

The documentation has not yet been updated (issue #110).

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