Perform operations on SI units

# SI Units

This is a library used to symbolically manipulate SI units. It comprises a `BaseUnit` type, used for base SI units like `kilogram`, `meter` etc, a `DerivedUnit` type, for units derived from them, and a `Composite` type, for when numerical coefficients are included. Python version 3.7 or greater is required. There are no dependencies.

This library is useful for dimensional analysis. It relies on the concept that SI units are all composed of varying exponents of 7 base units. We can think of all other units as linear combinations of base units.

You can perform normal multiplication, division, and power operations on units provided by this lib, and between them and `int`s and `floats`. You can check equality, based on the resulting combination of base units.

## Install

```pip install siunits
```

In Linux, you may need to use something like `pip3 install siunits`, or `python3.7 -m pip install siunits`.

## Examples

A base unit:

```import siunits as u

u.kg
>>> kilogram (kg), mass
```

A derived unit:

```u.v
>>> volt (V), [kg: 1, m: 2, s: -3, A: -1]
```

Multiplication:

```u.kg * u.s
>>> kilogram·second (kg·s), [kg: 1, s: 1]
```

Division:

```u.j / u.m**2
>>> joule / meter² (kg·s⁻²), [kg: 1, s: -2]

u.s**2 / u.kg**3
>>> second²·kilogram⁻³ (s²·kg⁻³), [s: 2, kg: -3]
```

With numerical coefficients:

```2*u.a**2 * 3*u.v**2
>>> 6kg²·m⁴·s⁻⁶
```

Equality testing is based on composition of base units:

```u.w == u.v * u.a == u.kg * u.m**2 / u.s**3
>>> True
```

## Todo

• Infer composite types that match a given custom (eg multiplied) unit
• Apply arithmetic to quantities as well (eg `energy`, `work` etc), and display them

## Scope

This project doesn't aim to provide conversions with other unit systems, like `cgs` and `imperial`, nor does it provide physical constants. For these tasks, try scipy.constants.

## Why add another unit library?

The clean symbolic manipulation this library provides doesn't appear to exist in any existing one.