Service interval for appointments should reflect actual service interval

Hi there, I wanted to submit a patch that I think will improve the plugin.

This is for Appointments+ 1.2.8, line 3203.

Replace this:

$step = 15 * 60; // Timestamp increase interval to one cell ahead

With this:

$service = $this->service;
$service_obj = $this->get_service( $service );
$step = $service_obj->duration * 60; // Timestamp increase interval to one cell ahead

We have services that are booked with different lengths (e.g. most are 15 minutes and one is 20 minutes) and of course those are incompatible schedules. We needed to be able to book the 20 minute one appropriately.

With this patch, get_timetable() shows the service with 20-minute appointments in 20-minute intervals, and the rest are 15.