use GetType<std::string> consistently

Podhorszki, Norbertrequested to merge
github/fork/germasch/pr/gettype into master

Created by: germasch

Sometimes we use GetType<std::string>, sometimes just "string". The code generally uses GetType<T>() instead of hardcoded strings for the other types, so this patch consistently does the same for strings.

Also fixes one external use from helper::GetType to GetType.

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