Move Quantity() methods to Link

[alextanski]

Everything that currently happens inside the Quantity() class should in fact be based on the Link object itself to gain flexibility and get rid of the middle man.

[alextanski]

Status update, ported and updated are...

• bases can be computed as effective version on both axis [new method: _margin(axis, effective)]
• missingfy() can be applied on both axes, base dropping works on both axes and total is correct
• _mean() can be applied on both axes and is vectorized
• _dispersion() stats can be applied to both axes and is vectorized
• combine() works together with arbitrary missingfied axes, broadcasts and auto-updates bases
• new _filter() method that can filter an existing Link on an arbitrary qp-logical condition (not completely finished for all logics, but this is straightforward)

[alextanski]

There is a .swap() method in the Link class: It should by default filter the swapped Link on the replaced axis' variable answercount, like this:

link.swap(axis='y', var='q5_1')
link.filter({'q8': has_count([is_ge(1)])})

[alextanski]

This can be abandoned for now and should get picked up at a later stage. The .filter() method is implemented in relation to #86 and #231.