Implement PERCENTILE_DISC in DAX

[stephanv]

SQL's PERCENTILE_DISC function returns a value from a dataset (see https://docs.microsoft.com/en-us/sql/t-sql/functions/percentile-disc-transact-sql), but doesn't seem to have an equivalent in DAX. DAX's PERCENTILE.INC *seems* to return the same result as SQL's PERCENTILE_CONT, but DAX's PERCENTILE.EXC doesn't return the same result as SQL's PERCENTILE_DISC. My team seems to prefer the result of PERCENTILE_DISC over PERCENTILE_CONT, hence my request...