While we might never be able to have 100% of our code strict, we can at
the very least do it for all of our tests. This ensures that our tests
are using our API with the types as intended by the test author, and not
silently be cast to what our API requires.
* Extract Percentile-type functions from Statistics (e.g. PERCENTILE(), PERCENTRANK(), QUARTILE(), and RANK())
* Unit test for PERCENTILE() with an empty (of numbers) dataset
* Extract Permutation functions from the Statistical class into a dedicated Permutations class
Retain the original methods in the Statistical class as stubs for BC, but deprecate them. They will be removed for PHPSpreadsheet v2
Note that unit tests still point to the Statistical class stubs; these should be modified to use the Permutations class directly when the stubs are removed
Also provided a basic implementationof the PERMUTATIONA() Function
Adrien Crivelli
Mark Baker