Convert StatutorySickPayCalculator into ActiveModel model

This will allow us to instantiate the calculator at the beginning of the flow
and set its attributes as we go along c.f. `PartYearProfitTaxCreditsCalculator`.

This commit extracts logic from the constructor into separate methods which are
only called on demand. This should make it easier to unit test these methods,
because we'll only need to supply the attributes that are actually needed for
the method under test; previously we had to supply many/all of the attributes to
avoid an error in the constructor.

lib/smart_answer/calculators/statutory_sick_pay_calculator.rb

@@ -1,16 +1,27 @@
 module SmartAnswer::Calculators
   class StatutorySickPayCalculator
-    attr_reader :waiting_days, :normal_workdays, :pattern_days
+    include ActiveModel::Model
 
-    def initialize(prev_sick_days:, sick_start_date:, sick_end_date:, days_of_the_week_worked:)
-      @prev_sick_days = prev_sick_days
-      @waiting_days = (@prev_sick_days >= 3 ? 0 : 3 - @prev_sick_days)
-      @sick_start_date = sick_start_date
-      @sick_end_date = sick_end_date
-      @pattern_days = days_of_the_week_worked.length
-      @normal_workdays_missed = init_normal_workdays_missed(days_of_the_week_worked)
-      @normal_workdays = @normal_workdays_missed.length
-      @payable_days = init_payable_days
+    attr_writer :prev_sick_days, :sick_start_date, :sick_end_date, :days_of_the_week_worked
+
+    def waiting_days
+      @prev_sick_days >= 3 ? 0 : 3 - @prev_sick_days
+    end
+
+    def pattern_days
+      @days_of_the_week_worked.length
+    end
+
+    def normal_workdays_missed
+      @normal_workdays_missed ||= init_normal_workdays_missed(@days_of_the_week_worked)
+    end
+
+    def normal_workdays
+      normal_workdays_missed.length
+    end
+
+    def payable_days
+      @payable_days ||= init_payable_days
     end
 
     # define as static so we don't have to instantiate the calculator too early in the flow
@@ -98,11 +109,11 @@ def weekly_rate_on(date)
     end
 
     def max_days_that_can_be_paid
-      (28 * @pattern_days).round(10)
+      (28 * pattern_days).round(10)
     end
 
     def days_to_pay
-      @payable_days.length
+      payable_days.length
     end
 
     def sick_pay_weekly_dates
@@ -123,7 +134,7 @@ def weekly_payments
     def weekly_payment(week_start_date)
       pay = 0.0
       ((week_start_date - 6)..week_start_date).each do |date|
-        pay += daily_rate_from_weekly(weekly_rate_on(date), @pattern_days) if @payable_days.include?(date)
+        pay += daily_rate_from_weekly(weekly_rate_on(date), pattern_days) if payable_days.include?(date)
       end
       BigDecimal.new(pay.round(10).to_s).round(2, BigDecimal::ROUND_UP).to_f
     end
@@ -146,9 +157,9 @@ def init_normal_workdays_missed(days_of_the_week_worked)
 
     def init_payable_days
       # copy not to modify the instance variable we need to keep
-      payable_days_temp = @normal_workdays_missed.dup
+      payable_days_temp = normal_workdays_missed.dup
       ## 1. remove up to 3 first dates from the array if there are waiting days in this period
-      payable_days_temp.shift(@waiting_days)
+      payable_days_temp.shift(waiting_days)
       ## 2. return only the first days_that_can_be_paid_for_this_period
       payable_days_temp.shift(days_that_can_be_paid_for_this_period)
     end