Optimization: read population data only if needed?

[alhom]

Here's a bunch of reads happening that might not be needed: https://github.com/fmihpc/analysator/blob/16b388fc5c6a7bdb09959de61e5ed5ec92427d8f/pyVlsv/vlsvreader.py#L282

Perhaps a general solution could be to read parameters from a cached version of the XML footer instead? But that'd require moving the metadata to be actually in the footer, not in the offsets. Bleh.

[ykempf]

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[ykempf]

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