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20130928 (Saturday, 28 September 2013)
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Clearable accounts
------------------

I first needed to do some vocabulary research. 
Imagine some contract which 
causes monthly anticipated payments followed by a quarterly invoice 
which leads to an adjusting payment:

=================== ====== ====== =========
Transaction         Debit  Credit Match 
=================== ====== ====== =========
January payment     100           X
February payment    100           X
March payment       100           X
March invoice              655.75 X
Adjusting payment   355.75        X
April payment       200
=================== ====== ====== =========

At some moment the system (automatically) or the user (manually) must 
insert those Xes in the in the `Match` column
to indicate that these movements should be "grouped" together 
and that the March invoice is *paid*.

In French this action is called "Lettrage des écritures comptables",
in (Belgian) German I call it "Buchungen lettrieren".

According to `Jack Dunwell <http://www.proz.com/translator/802595>`_ 
in a discussion thread at
`proz.com <http://www.proz.com/kudoz/french_to_english/accounting/3476207-lettrables.html>`_
this is called 
"Cross referencing the invoice against its clearing credit notes and cash."
Merci, Jack.


I'd extrapolate this to say that a "clearable account" is an acccount 
which is expected to require this action.
Not every account is clearable.

*Partner* accounts are clearable per se: if you pay or receive some money 
to or from a business partner, then you must somehow show why you did 
that.

But also *general* accounts can require clearing. For examople:

- 4110 T.V.A. due
- 4111 T.V.A. déductible
- 5800 Virements internes

That's why Lino now has an attribute "Clearable" "Lettrable" ("Lettrierbar") 
per general account to designate such accounts.
(I had a temptation to solve this by adding a field `partner` to 
`accounts.Account`. If this field is non-empty, the account 
is "clearable".
But that would be overkill and difficult to explain.)

New tables DuePaymentsByAccount.

Extended the Match class so that it works also per account. 
Which has the beneficient side effect af no longer ignoring the account 
when building the matches for a partner. Imagine for example a partner 
who is both customer and supplier and for whom we had exactly three 
operations::

  Sales invoice      1000
  Puchases invoice         700
  Payment                  300

Lino should not allow to clear this as is, but should require to add
a journal entry "Clearing"::

  Sales invoice      1000      S
  Puchases invoice         700 P
  Payment                  300 P
  Clearing                1000 S
  Clearing           1000      P


TODO: 

- context menu in row
- selecting more than one row
- adapt tests because number of fields has changed
- be able to fill from DocItems
- ...

I checked in nevertheless because it seems to work and is so beautiful!


Actions of the :menuselection:`Site --> Info` window
-----------------------------------------------------

The :menuselection:`Site --> Info` window (:class:`about.About <lino.modlib.about.models.About>`)
had a useless action "Detail", and clicking the Print button there
caused an internal server error.
Fixed.
:class:`BasePrintAction <lino.mixins.printable.BasePrintAction>` now overrides
:meth:`attach_to_actor <lino.core.actions.Action.attach_to_actor>`
to disable itself when :mod:`lino.modlib.system` is not installed.

Worked a bit on :lino:`/user/templates_api`.