add attr-acessor for selected, in order to be able to filter form selected...

add attr-acessor for selected, in order to be able to filter form selected volunteers on select in form

app/models/semester_process_volunteer.rb

 class SemesterProcessVolunteer < ApplicationRecord
+  attr_accessor :selected
+
   belongs_to :volunteer
   belongs_to :semester_process
   belongs_to :responsible, -> { with_deleted }, class_name: 'User',