Removed taxonomy and categoryFields parameters from find product by ID endpoint.

Now category taxonomy for a product is returned by default if the product fields are empty.
If the field list contains "categories" or "*" it returns the taxonomy.
Refactor to product model so it stores an array of Category, instead of an array of category ids.
Taxonomy data is now part of the product in the response, instead of a different key/value pair in the Json.
Taxonomy calculation is now supported for multiple product ids.