Make pint an optional dependency

[eltos]

Pint is used for

• unit conversion
• unit formatting
• automatic unit resolving of derived quantities

This makes pint an optional dependency. If not available

• Attempting unit conversion will fail (the default unit conversions are disabled, e.g. particle coordinate x defaults to units of m if pint is not available instead of mm if pint is available)
• Unit formatting uses a simplified fallback method
• Unit resolving will fail

The docs are updated to recommend installation as

pip install xplt[full]

and this is also shown to the user upon failure