Improving Builds in Cello Layer

[mabruzzo]

To further improve the robustness of the build-system, it would be useful to transition src/Cello/CMakeLists.txt away from using glob statements.

• If we could totally remove globs, then I think you would never need to destroy your build-directory (e.g. when swapping branches)
• you would still need to make a new build-directory if you changed something fundamental, like the precision

The primary drawback to this: there are a lot of files. Thus, the main src/Cello/CMakeLists.txt file would become unwieldy.

What if we moved the subcomponents into subdirectories?

• the renaming would be straight-forward. For example, mesh_Block.hpp -> mesh/Block.hpp
• We could pursue a gradual step-by-step procedure similar to what we have already done for the enzo-layer. I think this worked out relatively well for minimizing merge conflicts.

@jobordner do you have strong feelings about this? If you strongly prefer the existing structure, I'm happy to leave it alone.