Make "tier" a known attribute as integer

[danielhuppmann]

We often use tier as a variable-attribute to indicate importance of a variable - tier 1 is mandatory to be reported in a scenario-comparison project, tier 2 and 3 are optional.

This could be added as a standard attribute (as integer) to the VariableCode, but with a tag-list twist (a very special case of #402).

• If the Variable has a tier and the tag-list doesn't: easy
• If the Variable has a tier and the tag-list has a tier-attribute of the type "+1" - add the value to the tier.
• If the Variable doesn't have a tier: ignore

[I suggest the format "+1" to clearly indicate that this is added. If the tier in a tag-list is an integer, an error should be raised.]

This would help in cases like here where Final Energy|Non-Energy Use|Chemicals|Liquids should have tier 2 but Final Energy|Non-Energy Use|Chemicals|High Value Chemicals|Liquids|Biomass should probably be tier 4. The of these variables are made of the same tag-lists (each combining different priorities), so having this flexibility to indicate priority within a tag list would be very helpful...