Add PI Radians Unit

[Advay17]

Useful for reducing the number of imports necessary in files, and is cleaner than using std::numbers::pi every time in C++.

[calcmogul]

I'm not sold on the utility of this. You can just do the following in your local scope to reduce typing.

using std::numbers::pi;
using units::radian_t;

radian_t{pi}

We aren't worried about the extra typing in the library itself.

Also, if you really need units of pi radians, that's just units of half turns, so you could use units::turn_t instead of making a new unit type.

[Advay17]

It's mainly just syntactic sugar similar to how the java units library has both rotations and revolutions, to simplify usage for teams if they do choose to use it

[Advay17]

/format

[calcmogul]

Again, C++ already has a unit for that: units::turn_t. We don't need a second, less obvious one.