Extend possible values for the commodity_classification arg about comtradr HOT 3 CLOSED

this touches on #52, function ct_get_commodity_tables (could be an internal helper function only) takes valid comtrade parameters for the product classification and returns the reference tables to include for validation

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this implemented with 2f50898

However, we need to implement a check for max url length. It seems that the one for 2000 characters is implemented, as more than 50k characters returns an error (e.g. all HS codes) https://www.sistrix.com/ask-sistrix/technical-seo/site-structure/url-length-how-long-can-a-url-be

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This seems to be implemented on the Comtrade site with a check for the size of the URL request and caps at 4,096 characters. We check for this now too: 1a28183

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