Ensure relations are in order; this is still using a string comparison so not totally reliable.

--HG--
branch : 1.9
Tests no longer failing thanks to corrected sheet ordering.

--HG--
branch : 1.9
Move detect_worksheets in workbook.py and remove constants no longer used.

--HG--
branch : 1.9
Refactor functions to accept archive/Excel file rather than just part of it. Reduces coupling between functions.

--HG--
branch : 1.9
Remove read_sheet_titles (replaced by read_sheets) and fix tests.

--HG--
branch : 1.9
Stanadardise on using worksheet_path to identify a worksheet.

--HG--
branch : 1.9
Use the constant for the „xl“ prefix.

--HG--
branch : 1.9
Remove unused imports.

--HG--
branch : 1.9