toRadian(), toDegree, and mathematical constants

[jslee02]

This PR replaces the macros for mathematical constants with type safe template functions, and addes utility functions, toRadian(), toDegree(), for degree-radian conversion.

This resolves #314.

[mxgrey]

I like the idea of this pull request, although constants<T> could get somewhat annoying, especially since I think most of the time we will want to use constantsd anyway.

What if we also provided user-defined literals for pi and degrees? For example:

namespace dart {
namespace math {

constexpr double operator"" _pi (double x)
{
  return x * constantsd::pi();
}

constexpr double operator"" _deg(double x)
{
  return x * constantsd::pi() / 180.0;
}

} // namespace math
} // namespace dart

[jslee02]

I like the idea of this pull request, although constants could get somewhat annoying, especially since I think most of the time we will want to use constantsd anyway.

Right. I also considered providing only double version rather than templating. I don't have a strong preference here so I'm fine with that too.

Here are some other alternatives we could consider:

• If we provide only double version, this template struct can be just namespace and each item would be variable instead of function as:

namespace constants {
constexpr double pi = 3.14xxx;
}

auto twoPi = 2.0 * constant::pi;  // not constant::pi()

• We could consider shorter name like const::pi() or const::pi.

What if we also provided user-defined literals for pi and degrees?

Sounds really good. By the way, _deg and _rad seem to make sense to me, but I'm not sure if _pi is also good practice.

Sorry @mkoval @psigen, we're going to use another C++11 feature. 😈

[mxgrey]

I'm happy with this pull request. I think we can merge once the CI tests pass. 👍