Treat multiple input files as a single unit

[geoo89]

Previously, there were limits to what extent multiple input files could cross-reference each other. Now, cross-referencing has well-defined behavior. Similarly, if a sheet is defined multiple times, the parsing order determines which of the sheet will be used (the last one in the parsing order).

Fixes #104

[istride]

It would simplify things a lot if we defined a CSV workbook to be a directory containing CSV files.

workbook
├── content_index.csv
├── sheet1.csv
└── sheet2.csv

This should be considered the same as an XLSX file or Google Sheets document with three sheets/tabs. A CSV reader would then be identified by its base directory, so two CSV readers with the same base directory are identical.