Add IsExcluded('attribute') function to AF Analysis.

Add new isexcluded(attribute_reference) function to be able to easier distinguish between Tag/Attribute errors and excluded attributes.

Currently it is quite tricky to filter if an attribute is in error due to data source error or an excluded tag. It required multiple if then else clauses and BadVal functions to achieve this. An IsExcluded function would be a lot more practical and clear.