References/Sources in definitions file

[hgrecco]

Inspired by a comment in #1912, there is a proposal to include a references or sources for definitions. This is important for documentation and traceability reasons. This discussion is to bikeshed the actual format and implementation.

0. Do we call this source or reference?. I will use source but this is open.
1. Should this be an inspectionable property of each definition, available at runtime? If YES.
   This would require first a public API to access definitions (which I think is a good idea!).

>>> ureg.meter
<Unit('meter')>
>>> ureg._units["meter"]
UnitDefinition(name='meter', defined_symbol='m', aliases=('metre',), 
converter=ScaleConverter(scale=1), reference=<UnitsContainer({'[length]': 1})>, 
start_line=109, start_col=0, end_line=109, end_col=28, raw='meter = [length] = m = metre')

Such definition could have a source attribute.

3. Should we add a source to each definition file?
   This source will be applied to all definitions within that file.
   PRO: it is super easy to read and implement.
   CON: would require a definition file for each source.

I think it would be good to list the sources first.

4. Should we add a source to each definition?
   PRO: very flexible, only one definition file is needed.
   CON: it clutters the definition file and requires more time to implement

[hgrecco]

It would be great if we can compile here the references.