Add Normalisation

Add a `Normalise(): String` method.

Scenario: Say you have a length instance of `78'167 μm`. This is an unwieldy representation for humans to read. More natural forms of the same length would be `78.167 mm` or `7.817 cm`, where the latter value has been rounded.

Tasks:
- Find a good name for that method ("Normalised", "DisplayValue", "HumanReadable", etc. could be other names)
- Decide whether the value should or should not be rounded to max. three digits
- Find an implementation that efficiently computes the normalization for values in the **metric system**
- For the imperial system decide whether:
- To *not* implement this functionality
- To convert the value to a metric value that can be normalised
- To convert the value to the "best" representation within the same system (This may open up a can of worms...)

Note also the example in the readme: 3'456 Km/d => 40 m/s
- Is that a good normalization? (people are more used to Km/h)
- Compound Quantities will be hard to reasonably normalise...

Start by reviewing the existing quantity and unit APIs, then inspect the README example using 3'456 Km/d. The issue leaves the method name, rounding rules, imperial behavior, and compound-quantity handling undecided; done would require an agreed design before implementation and validation.

Written by the indexing model from the issue text.