Implement fluid interface in all entities

[Richtermeister]

Currently the fluid interface of entities is inconsistent. This PR fixes this and adds missing phpdocs as well.

(Also fixes a little bug where total shipping weight is not returned properly.)

[Richtermeister]

While on subject, should we add spec tests to make sure fluid interface is implemented on every setter? Could this be one test, or one per method?

it_implements_fluid_interface($var1, $var2)
{
    $this->setVar1($var1)->shouldReturn($this);
    $this->setVar2($var2)->shouldReturn($this;
}

Also, which phpdoc return should be used? @return self or @return $this or @return FQCN ?

[cordoval]

this is all $this

[pjedrzejewski]

Multiple models already have specs for that, I've put them all in 1 example, but I don't feel like it's smart to sit and add all these specs now. We can add them during refactoring or when things break. Thank you Daniel! 👍