STAT check: no duplicate Axis Values

[josh-hadley]

Per the OpenType Specification:

No two tables should provide information for the same combination of axis values:

• There should not be a format 1 table with axisIndex and value that match the axisIndex and value of a separate format 3 table.
• There should not be a format 1 table with the same axisIndex as a separate format 2 table if the format 1 value is greater than the rangeMinValue and less than the rangeMaxValue of the format 2 table. The format 1 value may be the same as the minimum or maximum of the format 2 range, but the format 2 nominalValue should be different.
• Since format 2 and format 3 each provide additional and distinct information associated with an axis value, a font may contain a format 2 table and a format 3 table with the same axisIndex where the format 2 nominal value is the same as the format 3 value. The flags and valueNameID of the two tables must be the same.
• A format 4 table may specify an axis value that is also specified in another table, but only if there is some other difference in the combination of axis values specified by those two tables.

There does not appear to be any check for these conditions in the fontbakery codebase.