No data status

Hi,

This pull_request has one purpose: add the status NO_DATA to the assessors. When an assessor is created, if the has_inputs() return 1, status will be NEED_TO_RUN, if return -1; status will be NO_DATA, if return 0, status will be NEED_INPUTS.

should_run() for a session should be set to return True and should always create an assessor.