AmpGen: Include what you use

In preparing for whitespace only changes by applying clang-format, I discovered that many include files are missing; so if you reorder the includes, it breaks. This should fix that or at least improve on that by running include-what-you-use (added info to the developer section of README).

The only changes are to the README, the includes, and the forward declarations. No functional changes.

For next MR:

Remember to run git ls-files '*.cpp' '*.h' | xargs clang-format -style=file -i before committing (or add it to your git hooks).

Use git blame -w -M when running git blame to ignore whitespace and moving code changes (Don't blame me for everything, in other words!)