[jesusVMayor]

Depends on #463 (children capacity in rooms), which this builds upon. Review
that one first; only the last commit belongs here.

A pricelist gave a room type one price per night, whoever stayed in it. Selling
a double for less when a single guest takes it, or charging for a third one,
could only be done by hand on every reservation.

What this does

Room types get two things: the occupancy their rooms guarantee (max_occupancy,
the smallest capacity among them, as any of them can be the one assigned) and
the occupancy their price is given for (default_occupancy, which defaults to
the former and follows it down when a smaller room joins the type).

A pricelist then carries, per room type, how much each guest above or below that
default changes the price, as a percentage or as a fixed amount:

default_occupancy = 2, price = 100

1 adult   ->  decrease 25%     ->   75
2 adults  ->  the price as is  ->  100
3 adults  ->  increase 10      ->  110
4 adults  ->  increase 10 x 2  ->  120

The modifier is linear, applying once per guest of difference, which is what
channel managers derive from a default occupancy as well.

It stays out of the way

The modifiers do not live in product.pricelist.item, so neither the rule
resolution nor its ordering change: an item is still one price per pricelist,
room type and day. The price is derived afterwards, in a single place.

A price asked for without an occupancy, or a room type with no modifiers
configured, is the plain one of the pricelist. Everything that prices a room
type without guests in hand keeps working untouched.

Where it is wired

product.pricelist.item._compute_consumption_price() ends in
_apply_occupancy_price(), which hands over to pms.room.type. Reservations
pass the adults staying through pms.reservation.line._get_price_kwargs(), and
the booking engine lets you pick them per room type so the price shown is the
one you will pay.

Changing the adults leaves the prices of the nights as they are, the same as
changing the pricelist or the room type does: the form flags them as outdated,
and updating them prices the nights for the adults staying. Nights added
afterwards are priced for them already. A price is never rewritten behind the
back of whoever wrote it, such as a channel manager writing what an OTA sold.
The birthdate of the guests checking in never takes part in it: nobody reprices
at the desk because a guest had a birthday.