Automatically update currency rates

Following https://discuss.tryton.org/t/automatically-update-currency-rates/1922/2

assigned to @xcodinas

review286271002 updated at https://codereview.tryton.org/286271002/#ps282351003

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review286271002 updated at https://codereview.tryton.org/286271002/#ps274181002

I've uploaded review321901002 which is updated to tip and has a more modular design to allow to customize the source of the rates.

assigned to @pokoli and unassigned @xcodinas

New review321901002 at https://codereview.tryton.org/321901002/#ps300221002

For me, it is missing the pieces described in https://discuss.tryton.org/t/automatically-update-currency-rates/1922/5
This requires a kind of scheduler a little bit like in the sale_subscription module.

I've added the option to select the source and the frequency allowing both options to be customized.

Currently forex-python only provides a single value for a day, but if somebody implements other methods that provide multiple values we may add several options for the sources. For example:

- Service Name (Daily Average)
- Service Name (Daily Opening)

This allows us to keep our desing simple while supporting multiple options.

review321901002 updated at https://codereview.tryton.org/321901002/#ps318031002

review321901002 updated at https://codereview.tryton.org/321901002/#ps292131002

I have started a new review347191002 which include the latest remarks about the design. The design allow to have multiple frequency per currencies and even from different sources. The only advise is to always use the same base currency which I documented. In theory different base currency could work if each currency convertion possible is updated each time.

I also filled https://github.com/MicroPyramid/forex-python/pull/83 in order to ensure to get the proper rate for today.

assigned to @ced and unassigned @pokoli

review347191002 updated at https://codereview.tryton.org/347191002/#ps347201002

review347191002 updated at https://codereview.tryton.org/347191002/#ps349191002

review347191002 updated at https://codereview.tryton.org/347191002/#ps333831002

New changeset 2fb24d104afb by Cédric Krier in branch 'default':
Add scheduled task to update rates
https://hg.tryton.org/modules/currency/rev/2fb24d104afb

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closed