emissions_profile*() uses co2$values_to_categorize if present

[maurolepore]

Closes #603

This PR prepares tiltIndicator for when values_to_categorize come from "co2". When that happens the code will just use that column. Until then it'll continue to use the internally computed values_to_categorize.

Once https://github.com/2DegreesInvesting/tiltIndicatorBefore/issues/7 is done, then we can remove the code that computes values_to_categorize (#606).

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[kalashsinghal]

@maurolepore I just want to remind you that the column name will be profile_ranking and not values_to_categorize. I am mentioning to be sure its not a mistake. Thanks!

[maurolepore]

Thanks @kalashsinghal, better safe than sorry :-)

This PR was merged before we decided on the new name profile_ranking. But now all instances of values_to_categorize have been renamed to profile_ranking.

#610