Implement `AVERAGE(inventory)` BQL fucntion

[dnicolodi]

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I'm going through the TODO.md file checking what has been implemented already, what fits with the current vision for beanquery, and what remains to do. There are two entries related to an AVERAGE() or AVG() function that operates on inventories.

The first is an example query:

SELECT
  root_account, AVG(balance)
FROM (
  SELECT
    MAXDEPTH(account, 2) as root_account
    MONTH(date) as month,
    SUM(change) as balance
  WHERE
    date > 2014-01-01
  GROUP BY 
    root_account, 
    month
)
GROUP BY 
  root_account

which should work, except there is no AVG() function. How would an AVG() function work on inventories? I'm not sure a definition where the different commodity components of the inventory are averaged makes much sense, but I cannot think of a better one that is universally useful.

The second is this note:

FIXME: To render holdings at "average cost", e.g. when aggregating by account, you could provide an "AVERAGE(Inventory)" function that merges an inventory's lots in the same way that the holdings merge right now. THIS is how to replace and remove all holdings support.

which I'm not sure I completely understand.

@blais What did you have in mind?