This is intended for both local developer use and for CI.

This depends on GNU parallel. One-time install:

macOS: brew install parallel clang-format
Debian: apt install parallel clang-format-10

To format all ORCA / GPOPT code:

$ src/tools/fmt fmt

To check for formatting conformance:

$ src/tools/fmt chk

To modify the configuration, you'll need two steps:
1. Edit clang-format.intent.yml
2. Generate the expanded configuration file:

$ src/tools/fmt gen

This commit also adds a formatting README, To document some of the
rationale behind tooling choice. Also mention the new `README.format.md`
from both the style guide and ORCA's main README.