[QUANTITY] doc consistency

[Pierre Didelon]

Hi,

Pierre Didelon wrote:


> Another point concerning all the levels is related to serialisation. As you
> outlined it, it is  a very important point and I feel that a corresponding
> method would preferably included in the interface definition at each level.
> 

May I insists?
It would be nice to materialise the serialization possibility
of quantity by a method, appearing in clase Frame or BasicQuantity.
Moreover it would be nice if this "serialize" method could have a calling param
to choose the output format, this one could perhaps be pure_xml, VOTable,
ASCII, Fits, SQL Statements... or whatever needed or required, incrementally,
starting implementation of the most obvious one, which would be
the default format, if the required one by user is unavailable
Something like getSerialization(format: string): string

regards,
pierre