Instead of defining a period and duration for only one preventive
maintenance per equipment, you can define multiple preventive
maintenance kind for each equipment.

Maintenance Kinds have to be defined through the configuration menu.
Their name have to be unique and can be set as active or inactive,
should these not be used anymore.

On any equipment over the maintenance tab, the maintenance plan be
accessible, allowing to add different maintenance kind with their own
frequency and duration. The next maintenance date will then be computed
automatically according to the start's date and the frequency defined,
but the maintenance request won't be created automatically as is the
case in Odoo's Maintenance module. In the plan there's also a field
allowing the user to set the maintenance horizon, insert the
instructions to follow on the maintenance that will be forwarded to the
maintenance request generated from the plan.

This module uses the original Cron job of Odoo's Maintenance module to
generate maintenance requests. To do so, it takes into account the
planning horizon and generates all maintenance requests whose schedule
date would fall inside that planning horizon. Therefore, the maintenance
manager can have a proper planning of how many maintenance requests are
programming for the future. Leaving planning horizon to 0 will only
create those maintenance request that are scheduled for today.

We can also create maintenance requests from a plan using a domain for
selecting the equipments. This way, we might have a single plan that
will generate all the requests. In order to use it, we need to mark the
Generate with Domain field.
