feat(semester-process): builder for creating SemesterProcessVolunteer

- takes query of elegible volunteers and then builds SemesterProcessVolunteer for each
- if array of volunteer ids is passed, it will only build SemesterProcessVolunteer for these

app/models/semester_process.rb

@@ -36,4 +36,12 @@ class SemesterProcess < ApplicationRecord
     end
   end
 
+  def build_semester_volunteers(volunteers, selected = nil)
+    volunteers = volunteers.where(id: selected) if selected
+    semester_process_volunteers << volunteers.to_a.map do |volunteer|
+      spv = SemesterProcessVolunteer.new(volunteer: volunteer, selected: true)
+      spv.build_missions(semester)
+      spv
+    end
+  end
 end