Remove duplication of work with getField selectors (if we stick with the current methodology)

[plocket]

I think we can avoid all these repetition in selector clauses by setting a variable, e.g., base_64_var_or_index to either a variable name or an index variable. We have the ability to detect whether a given argument var_name is a variable name or an index name. So once we know which one it is, we can just use that.

Originally posted by @niharikasingh in #160 (comment)

[plocket]

Great suggestion. We're now switching to getPageData(), so this is no longer necessary.