Fix a bug in PowerPreBasis<FlatLexicographic,...>::size()

• Add test case triggering the bug mentioned in !350 (comment 126349)
• Extend checkBasis() to do a complete consistency check of index tree and basis.size(prefix)` method.
• Fix the bug.
• Cleanup and simplify the various basis.size(prefix) implementations by
  • extracting a popFront() operation,
  • improving internal documentation,
  • modify prefixes in-place instead of doing almost exact copies.