DATAUP-698: Fix ?sv files with missing header items getting strange key names

[MrCreosote]

Pandas would replace missing header items with weird names like Unnamed: 2_level_0. Pandas has enough quirks that it's not worth using it - simpler to just use a low level csv parser.

[MrCreosote]

Pandas was probably a mistake for ?sv parsing, where the standard library parser works just fine and Pandas has a number of quirks to work around, but abstracting the differences in handling xlsx and xls files really simplifies the code, so it's probably worth the quirks. I'm going to take a look and see if there's better ways to work around those.

[sonarcloud[bot]]

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