Allow custom macro parameter

[dylanjm]

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Pull Request Description

closes #54

Adds a year_variable as a subnode to the time_discritization node.

Adds steps in a modifying function to add year_variable to GRO_dispatch groups.

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[PaulTalbot-INL]

All these changes look good; I suspect you'll need to either retrain the sine ARMA using "year", or add the year name node to the other test cases, or default to "Year" instead of "year". Any of the three is fine to me.

[PaulTalbot-INL]

Changes look good.