This allows method bodies to use the full Angle API, particularly
operators. Trivial getters have not been moved to minimize code
duplication.

Client code is generally simpler if wrap methods return an altered
Angle instead of modifying the object they are called on.

This change allows Angles to be used in compile-time expressions and
take advantage of noexcept optimizations. It also simplifies some
existing metaprogramming using C++11-only tools.

This commit ensures all API elements are documented, and that
documentation complies with current style guidelines.

These methods were added to Angle as part of
2b49698 to be used in table::Match.
They have not been used anywhere else, their use case is more specific
than other Angle operations, and they do not need any access to
Angle's internals (cf. Effective C++ 3rd. Ed., Item 23). In addition,
making them an implementation detail of Match reduces coupling between
afw::geom and afw::table, which may be useful in the long run.