Optimize VOTable building process

Currently, the VOTable output relies on Astropy (and numpy) : https://docs.astropy.org/en/stable/io/votable/

As some methods are missing (or misunderstood) in the API, the current workflow is :

• create a VOTable object in memory
• add a row to this table for each result
• writing the whole structure into a file
• reopen the file and read the content to serve it through the response object

The process could be more efficient using a file stream pattern