Added method to unfinish a visit as explained in #351. added properties to get next visit and future visits

Issue #292 and related to issue #195 . Considered update and create events as different cases. If updated, check all visit_numbers. Added tests accordingly

Issue #195 added tests and changed the post_save function for visit. I didn't saw a problem of concurrency but a problem of saving the visit accordingly. Maybe we need more tests

fixed problem with the duration of auto created follow up visits. relative date was in days instead of months

- Add field default_delta_time_for_control_follow_up to study
- Add field default_delta_time_for_follow_up_units to study
- Add field default_delta_time_for_patient_follow_up to study
- Alter field default_visit_duration_in_months on study
- Alter field default_voucher_expiration_in_months on study

address, city, country, data_born, default_written_communication_language, email, first_name, languages, last_name, phone_number, phone_number_2, phone_number_3, postal_code removed from study_subject (all this information is stored in subject)

the code is fixed so unit test passes