Support for deserialization

Hello,

Right now, only serialization is possible. Is there any reason deserialization is not implemented ?
I'm on a project where I want to extends the "safety" of units to the data in our database.
For example, I would like to be able to do that:
```
quantity l0 = 3.14 * si::meter;
stream << l0; // "3.14 m"

quantity a;
stream >> a; // throw an exception

quantity l1;
stream >> l1; // OK
```

Thank you!