Speedup structure parsing
Speedup structure parsing

- Structure data columns used to be merged with the Pandas structure DataFrame one at a time which is slow.
   Solved by constructing an intermediate DataFrame with the column selection and merging in one go with the main DataFrame
- Atom element types used to be assigned by iterating over the DataFrame which are many search/replace operations.
   Solved by extracting all atoms for non-assigned elements and their DataFrame index. Assign element types and merging
   back in one go using .loc on the index numbers.

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