Numbers cannot be defined using unit and scale aliases

**As a** user
**I want** to define a `Number` using `Unit` and `Scale` string aliases instead of an `Enum`s
**So I** can have flexibility when declaring calculations

```
Number(unit=Units.liter, scale=Scale.milli, quantity=2)
```
would become something like
```
Number('mL', quantity=2)
```