Log Message:
libixion, py-libixion: Update to 0.16.1

Changelog:
0.16.1
    fixed a build issue on 32-bit linux platforms, caused by slicing
    of integer string ID values.

    worked around floating point rounding errors which prevented
    two theoretically-equal numeric values from being evaluated as
    equal in test code.

0.16.0
    documentation
	added quickstart sections for using model_context and
	document classes.

	added code example for cell_access class.

    C++ API
	added new function to allow printing of single formula
	tokens.

	added method for setting cached results on formula cells
	in model_context.

	changed the model_context design to ensure that all sheets
	are of the same size.

	added an accessor method to formula_model_access interface
	(and implicitly in model_context) that directly returns a
	string value from cell.

	added cell_access class for querying of cell states without
	knowing its type ahead of time.

	added document class which provides a layer on top of
	model_context, to abstract away the handling of formula
	calculations.

	deprecated model_context::erase_cell() in favor of
	empty_cell().

    formula parser / interpreter
	added support for 3D references - references that contain
	multiple sheets.

	added support for the exponent (^) and concatenation (&)
	operators.

	fixed incorrect handling of range references containing
	whole columns such as A:A.

	added support for unordered range references - range
	references whose start row or column is greater than their
	end position counterparts, such as A3:A1.

	fixed a bug that prevented nested formula functions from
	working properly.

	implemented Calc A1 style reference resolver.

    misc
	formula results now directly store the string values when
	the results are of string type. They previously stored
	string ID values after interning the original strings.

	removed build-time dependency on spdlog.