[Feature Request]: unit_cast

When I try to cast a quantity to it's numerical representation I can use ` quantity_cast`. That's great. What I think could also be very helpful is a shorthand for casting a quantity to a different unit. Now I know that this can be done like so:
```
boost::units::quantity siLength;
auto cgsLength = static_cast>(siLength);
```

However this feels kinda more bulky than it'd need to be. It would be very neat if something like this would be possible:
```
boost::units::quantity siLength;
auto cgsLength = unit_cast(siLength);
```

So essentially I am asking for a way to cast to a different (compatible) unit without having to explicitly write that I want to cast it to a quantity again. That should be deduced. Thus when starting from a quantity of double, I'd expect this cast to automatically cast to a quantity of double as well. Same for int and all other types.

Would something like this be possible? :thinking: