Review TaxCategory and TaxScheme

The relations of TaxCategory and TaxScheme need some review..

1) The definition of TaxCategory is "A class to describe one of the tax categories within a taxation scheme (e.g., High Rate VAT, Low Rate VAT).", but the TaxScheme is within TaxCategory and not vice versa..

2) TaxScheme is mandatory within TaxCategory, but this relation is incompatible with some reporting/business customs as well as some code lists (for example how tax is reported in the U.S., and how X12 code lists represents taxes).