Implements RFC-261. Update DM C++ Style Guide (primarily section 6) to enable automatic code layout with clang-format.
This involves adding a version controlled LSST clang-format style that is as much as possible compliant with the current layout rules. And subsequently removing or modifying the existing layout rules to be compliant with the output created by clang-format using this style.

The intended end result is that users can simply run clang-format (with the LSST provided style) to automatically fix all C++ code layout issues.

Users are still free to not use clang-format as long as the output looks the same.