Use a recommended date format (xsd:DateTime) for clients and the server

[niio972]

For now, we use a wrong date time format (yyyy-MM-dd HH:mm:ssZ).
We must use the recommended format xsd:DateTime.
We must add 'T'separator indicating that time-of-day follows the first part of the date.
e.g. (yyyy-MM-dd'T'HH:mm:ssZ).

This change will impact the all API clients (R, Python, Web app), internal WS search method parameters and class models with datetime attribute.