Integration 1712

• initial changes to compile with clang and some general layout (code and directories).
• no functional changes

@andy : FYI we'd just need to adjust the Make/files targets (4 files) to make this into a module. All the base code is now wrapped into a single libavalanche, and we have 1 solver and 2 utilities, so not very many risks of collision. I thus figured there's no point in wrapping this into the Foam::Module namespace.

Looks good to me, please merge.

merged

mentioned in commit 6adf9536