Numbers cannot be instantiated using "unit aliases"

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