libixion, py-libixion: Update to 0.15.0

Changelog:
ixion 0.15.0

* fill_down_cells() method has been added to model_context, to fill duplicate
  values downward from an arbitrary cell position.  For now, it always
  duplicate the value of the source cell; cell value enumerations are not
  supported.  Also, filling down of formula cells is not yet supported.

* cell iterator has been aded to model_context, for efficient horizontal and
  vertical iterations of cell values in an arbitrary range within a sheet.

* improved the cmake-based build, with all test cases integrated.

* switched to spdlog for compile-time debug log outputs.

* improved performance by removing repeated fetching of each formula
  cell prior to interpretation.

* reworked topological sorting of formula cells to avoid having to
  unnecessarily build dependency relations twice; once during the
  dirty cell determination and twice prior to interpretation of
  formula cells.  The new code performs topological sorting during the
  dirty cell determination phase.

* reworked the cell reference tracker to use R-tree.  This allows more
  accurate tracking of references involving grouped formulas.

* fixed a resource leak with dynamic loading of compute engine modules.

(ryoon)