Provide additional method for getting join PK.
Force inclusion of PK/FK in certain circumstances

In case Include in List Query has been set to "No" for table PK or join
FK, then we force these to be included if any other element in the group
is included.
Hide symptoms of unset placeholders in data[0]

Missing placeholders when Merge Rows and Reduce Data were causing php
warnings to appear e.g.

Notice: Undefined property: stdClass::$fch_cat___id

in/homepages/3/d85014684/htdocs/fairmile/dev/joomla3/components/com_fabrik/models/list.php
on line 9455

This fix checks that the property exists as well as providing some
optimised property lookups.