FIX Don't assume a DataObject's table.

[NightJar]

Knowing if a DataObject has a table or not is irrelevant - data is queried
out and pushed into solr all the same, whether there is a join table, a
base table, or any combination with maybe a few no-tables in between.
SomeDataClass::get() will fetch all data irrelevant of tables, that's the
point of an abstraction like SilverStripe's ORM.

The index featured is a default install, where Page is defined as

use SilverStripe\CMS\Model\SiteTree;

class Page extends SiteTree
{

}

and defines an index adding only Page::class to it.

Resolves #168

[robbieaverill]

I like the reasoning but would like to test it before merging

[NightJar]

Seems fair :)

[dhensby]

wow - what is this about? what a bizarre constraint. I don't see that this could have any regression for existing code at all - this simply allows us to add all DO classes instead of an arbitrary subset of them.