Add the ability to "rollup" values in a time series (#554)

* Clean up time series models

* Add value group rollup class for summarizing hierarchical data

* Integrate new classes

* Update UI to use new patterns

* Update D3 charts to expect new data format

* Clean up account model

* More cleanup

* Money improvements

* Use new money fields

* Remove invalid fixture data to avoid orphaned accountables

* Update time series to work better with collections

* Fix tests and UI bugs

app/models/time_series/trend.rb

@@ -0,0 +1,48 @@
+class TimeSeries::Trend
+  attr_reader :type
+
+  # Tells us whether an increasing/decreasing trend is good or bad (i.e. a liability decreasing is good)
+  TYPES = %i[normal inverse].freeze
+
+  def initialize(current: nil, previous: nil, type: :normal)
+    validate_data_types(current, previous)
+    validate_type(type)
+    @current = current
+    @previous = previous
+    @type = type
+  end
+
+  def direction
+    return "flat" if @previous.nil? || @current == @previous
+    return "up" if @current && @current > @previous
+    "down"
+  end
+
+  def value
+    return Money.new(0) if @previous.nil? && @current.is_a?(Money)
+    return 0 if @previous.nil?
+    @current - @previous
+  end
+
+  def percent
+    return 0.0 if @previous.nil?
+    return Float::INFINITY if @previous == 0
+    ((extract_numeric(@current) - extract_numeric(@previous)).abs / extract_numeric(@previous).abs.to_f * 100).round(1).to_f
+  end
+
+  private
+    def validate_type(type)
+      raise ArgumentError, "Invalid type" unless TYPES.include?(type)
+    end
+
+    def validate_data_types(current, previous)
+      return if previous.nil? || current.nil?
+      raise ArgumentError, "Current and previous values must be of the same type" unless current.class == previous.class
+      raise ArgumentError, "Current and previous values must be of type Money or Numeric" unless current.is_a?(Money) || current.is_a?(Numeric)
+    end
+
+    def extract_numeric(obj)
+      return obj.amount if obj.is_a? Money
+      obj
+    end
+end