implement usecols

[MoiraZuber]

Implement pandas read_csv parameter usecols to only read selected columns from metadata in order to speed up the script and reduce memory usage.

TODO:

• [ ] Check list of columns for missing entries (In the line above, dtype is provided for columns that are apparently not used throughout the script?)

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[emmahodcroft]

I've run this on scicore and it runs without errors, so we're capturing all columns. Will merge in!