Issue #278 Avoid deleting availabilities from the daily planning and add more control for undefined values

Issue #276 changed check_nd_number_regex to receive a study and to filter out null or blank nd_numbers in the studySubjects

Issue #266 Fixes problem when no voucher is checked to be printed. Rises a tooltip. But, even if the call was produced and no template was found in generate_for_vouchers, it would be redirected to the previous page (subject edit) or, if no previous page is found, to the subject list

Fixes most of #263 : nd number should have PDP prefix; screening numbers should be filled in (using P-) ; location prefix should be defined: P (CHL), F (Flying team)