Optional group by time intervals to use with _date_ and _datetime_ fields

<!--
You can use a special interval on temporal -date and datetime- fields to alter 
the default behaviour of a temporal field search grouping. 

The ORM automatically groups the records by 'month', bu you have four other 
options: 'day', 'week', 'quarter' and 'year'.

I leave you an example on how to use these:
-->

<search string="Pickings">
    <group expand="0" string="Group By">
        <filter string="Dayly" domain="[]" context="{'group_by':'min_date:day'}"/>
        <filter string="Weekly" domain="[]" context="{'group_by':'min_date:week'}"/>
        <filter string="Monthly" domain="[]" context="{'group_by':'min_date'}"/>
        <filter string="Quarterly" domain="[]" context="{'group_by':'min_date:quarter'}"/>
        <filter string="Yearly" domain="[]" context="{'group_by':'min_date:year'}"/>
    </group>
</search>

<!-- 
You can see a working example here: 

https://gitlab.e-mips.com.ar/rafaela-alimentos/logistic_transport/blob/devel/views/delivery_view.xml#L69

And you can read the ORM code that allows us to do this here:

https://github.com/odoo/odoo/blob/8.0/openerp/models.py#L1956
-->