feat: added formula api to declare unit values

[dhfwc18]

The formula api accessible via declare_unit allows users to build a quantity from a numeric value and an SI unit expression (in string).

Example:

from si_units import declare_unit, KILO, WATT, HOUR
declare_unit(5.0, "kWh") == 5.0 * KILO * WATT * HOUR
True

This one is just a quality of life feature that I think would be quite useful. Also apologies if it seems like I did a massive edit on __init__.py I have auto-lint on save setup for my IDE and the linter dislike the non-alphabetically sorted imports. Aside from reordering the imports, all the edit realistically do is expose declare_unit.

[g-bauer]

Interesting addition. I wonder if it makes more sense to have this in quantity so that we can have unit-checked parsing from a string there as well (and in serde so that we can use SI units to parse JSON). I have a local branch that enables parsing via winnow.

I could clean that up and we can expose it to python. The interface could be the same as yours.

let p: Pressure     = "1.3 N/m^2".parse()?;
let p: Pressure     = "15 bar".parse()?;
let mw: MolarWeight = "15 g/mol".parse()?;
let v: Velocity     = "9.81 m/s".parse()?;
let a: Acceleration = "9.81 m/s^2".parse()?;

@prehner what do you think? Is this in scope for this crate?

[dhfwc18]

@g-bauer I was thinking about the same thing. Just thought these kind of string parsing is more of a Python (Pythonic?) thing -- unyt has a similar API, though it is overloaded under object init, which I always find to be really awkward (think this is more elegant).

I think this is more for Python projects where presentation to or collaboration with less programming-oriented stakeholders are required. You know, demo in Jupyter notebooks etc.

Not sure if the same API is as useful if you are working with a compiled language like Rust though. Supposed importing the const directly provides better type clarity than an adhoc declaration with strings. Really depends on your use case I suppose?

I leave this to your better judgement.