bugfix: LASSO tracer emission and registry issues
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Fixed issue with ptracer4 accidentally getting emissions in addition to ptracer3. 
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Moved LASSO tracer changes within the registry back to Registry.EM from Registry.EM_COMMON. This seems clumsy, but it solves an issue with the order of the tracer 4-D array definitions for the initial blank entry. 
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Capitalized names of PTRACER variables for the wrfout files. 
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Switched checks for tracer_opt to use LASSO_TRACER instead of a number.