Add the ability to "rollup" values in a time series

[zachgoll]

Another prerequisite PR for #543 that aims to make it much easier to work with time series and aggregated data.

Overview

This PR introduces two important domain concepts:

• TimeSeries + TimeSeries::Value
• ValueGroup

Time Series Data

Maybe will be presenting user data in a time series format often. The TimeSeries model accepts an array of TimeSeries::Value objects, which have a date and value. This is a generic domain concept that works with numeric and money values:

money_series = TimeSeries.new([ { date: 1.day.ago.to_date, value: Money.new(100) }, { date: Date.current, value: Money.new(200) } ])

regular_numeric_series = TimeSeries.new([{ date: 1.day.ago.to_date, value: 100 }])

It also works with collections via the from_collection class method:

balance_series = TimeSeries.from_collection @account.balances, :balance_money

Value Groups

The purpose of value groups is to recursively "roll up" both individual values and time series arrays based on an arbitrary hierarchy. For example, we can create a hierarchy of assets:

• assets
  • depositories
    • checking account
    • savings account
  • other assets
    • collectable account

# See value_group_test.rb
setup do
    checking = accounts(:checking)
    savings = accounts(:savings_with_valuation_overrides)
    collectable = accounts(:collectable)

    # Level 1
    @assets = ValueGroup.new("Assets")

    # Level 2
    @depositories = @assets.add_child_node("Depositories")
    @other_assets = @assets.add_child_node("Other Assets")

    # Level 3 (leaf/value nodes)
    @checking_node = @depositories.add_value_node(checking)
    @savings_node = @depositories.add_value_node(savings)
    @collectable_node = @other_assets.add_value_node(collectable)

    # Can attach time series to one or more nodes (which will also be "rolled up")
    @checking_node.attach_series(TimeSeries.from_collection(@checking.series, :balance_money))
end