[ADD] budget by account

[sbidoul]

This introduces a new data source: MIS Budget by Account, next to the existing MIS Budget by KPI.

This essentially includes OCA/mis-builder-contrib#3 in mis_builder_budget with the subtle but important difference that budget items now cover a period instead of a single date. A pro-rata temporis adjustement is made on the budget items to correspond to the reporting periods.

Common parts between MIS Budgets by KPI and Account are moved to abstract base classes.

This PR includes refactorings from [ ] #258. The last 3 commits are relevant here:

• add the mis.budget.by.account and mis.budget.by.account.item models, and instroduce the abstract base classes
• add the prorata.read_group.mixin model that does the pro-rata-temporis adjustments in read_group and emulate a searchable date field based on date_from and date_to.
• add the new MIS data source

I leave as a future improvement the capability to mix budgets by KPI and by account.

[fclementic2c]

functional test ok

[sbidoul]

Merging into the 3.6 staging branch. People interested to test further can do so in #254

/ocabot merge

Thanks for testing, @fclementic2c

[OCA-git-bot]

This PR looks fantastic, let's merge it!
Prepared branch 10.0-3.6-staging-ocabot-merge-pr-259-by-sbidoul-bump-no, awaiting test results.

[OCA-git-bot]

It looks like something changed on 10.0-3.6-staging in the meantime.
Let me try again (no action is required from you).
Prepared branch 10.0-3.6-staging-ocabot-merge-pr-259-by-sbidoul-bump-no, awaiting test results.

[OCA-git-bot]

Congratulations, your PR was merged at cb60ce5. Thanks a lot for contributing to OCA. ❤️