Allow unused definitions

[Tehforsch]

The unit_system macro generates the type aliases corresponding to the dimensions and the Quantity constructors corresponding to the units for every storage type. It can happen quite quickly that one defines a dimension/unit that is only used for one of the storage types. In that case, a confusing unused code warning will be emitted if the module which calls unit_system is not a public export of the current crate. Minimal example:

use diman::unit_system;

unit_system!(
    quantity_type Quantity;
    dimension_type Dimension;

    dimension Length;

    #[base(Length)]
    unit meter: Length;

);

pub fn main() {
    let x = self::f32::Length::meter(5.0);
}

With f32 and f64 features enabled, this will generate an error

warning: type alias `Length` is never used
 --> src/my_mod.rs:7:15
  |
7 |     dimension Length;
  |               ^^^^^^

I only see two simple solutions to this problem:

1. Add #[allow(unused)] for every dimension / unit constructor.
2. Require users to export the Quantity struct by making the module which calls unit_system a public part of the library.

Now 2. is how it currently is, but is clearly not a good solution. Implementing 1. is fine in principle, but it is a bit unfortunate that this rules out getting useful warnings that could identify unused dimensions/units.