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

Carsten Gräserrequested to merge
bugfix/fix-power-size-flatlexicographic into master
  • 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.

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