Reorder properties, magic methods, public and private methods in classes

**Is your feature request related to a problem? Please describe.**
Ordering of properties, magic methods, public and private methods is not standardised.

**Describe the solution you'd like**
New standard for structuring a class is as follows:
1. ```__new__```
2. ```__init__```
3. Other magic methods (__)
4. Properties
5. Public methods
6. Private methods
This order is because magic methods, properties and public methods make up the interface of the class. Properties must precede public methods so that Sphinx creates them directly after Attributes.

**Classes which require refactoring:**
common.units:
- UnitFloat
- UnitNDArray