Alternative date/time selection method for the request

[smithara]

Currently one can only make requests specified by the start and end datetime. It might be useful to be able to pass a list of desired datetimes, which could be implemented by wrapping many requests or by adding a new WPS process on the server to handle this. Similarly, one may want to directly exclude some periods from the request - see Cleaning in https://github.com/ESA-VirES/VirES-Python-Client/wiki/Feature-list

This is not so important as it can readily be achieved by the user in other ways.