Implemented legacy vat handler

> Pushing on behalf of @crevillo 

Here is the vatHandler. Seems to be working. 
Some things to note

I needed to change converter because we were dealing with dataText[0] for the isVatIncluded thing, but that value is not the first but the second.

Created two new Exceptions class
One is general so every Gateway can't use for cases when VatRate is not found
Other is "Legacy" specific, for the case when vat automatic is used
Adds an `ezprice_value` twig function that transforms a `Price\Value` to a `PriceWithVatData`, that aggregates the original price data with `priceIncludingVat` and `priceExcludingVat` properties.
`2` is the value for Price Ex Vat option when you include prices from the admin interface. Ref. https://github.com/ezsystems/ezpublish-legacy/blob/master/design/standard/templates/content/datatype/edit/ezprice.tpl#L18