☂ Statistics streamlining

[Jolanrensen]

Continuation of #558 which fixed the most annoying bugs related to describe.

See #558 for more information.

Our statistics functions need some more love. We used to have many missing types (mostly fixed by #937), but there are yet some more inconsistencies to be solved:

As mentioned here #543, some functions like median(ints) might result in an unexpectedly rounded Int in return. It might be better to let all functions return Double and then handle BigInteger / BigDecimal separately for now, as they're java-specific for now.

There are plenty of public overloads on Iterable and Sequence. It's fine to have them internally, but I feel like we're clogging the public scope here. mean, for instance, is already covered in the stdlib.

We'll need to hide public functions that are not on DataColumn as @AndreiKingsley will probably make a statistics library for that anyway.

We need to honor some conversion table (see below)

Function	Conversion	extra information	nulls in input
mean	Int -> Double		All nulls are filtered out
	Short -> Double
	Byte -> Double
	Long -> Double
	Double -> Double	skipNaN option, false by default
	Float -> Double	skipNaN option, false by default
	BigInteger -> BigDecimal?	null instead of NaN in output
	BigDecimal -> BigDecimal?	null instead of NaN in output
	Number -> Double	skipNaN option, false by default
	Nothing / no values -> Double (NaN)
sum	Int -> Int	All default to zero if no values	All nulls are filtered out
	Short -> Int
	Byte -> Int
	Long -> Long
	Double -> Double	skipNaN option, false by default
	Float -> Float	skipNaN option, false by default
	BigInteger -> BigInteger
	BigDecimal -> BigDecimal
	Number -> Double	skipNaN option, false by default
	Nothing / no values -> Double (0.0)
cumSum	Int -> Int	All default to zero if no values	All can optionally skip nulls in input with skipNull option, true by default
	Short -> Int		important because order matters with cumSum
	Byte -> Int
	Long -> Long
	Double -> Double	skipNaN option, true by default
	Float -> Float	skipNaN option, true by default
	BigInteger -> BigInteger
	BigDecimal -> BigDecimal
	Number -> Double	skipNaN option, true by default
	Nothing / no values -> Double (0.0)
min/max	T -> T? where T : Comparable<T>	For all: null if no elements	All nulls are filtered out
	Int -> Int?
	Short -> Short?
	Byte -> Byte?
	Long -> Long?
	Double -> Double?	If has NaN, result will be NaN, needs skipNaN option?
	Float -> Float?	If has NaN, result will be NaN, needs skipNaN option?
	BigInteger -> BigInteger?
	BigDecimal -> BigDecimal?
	Number -> Double?	If has NaN, result will be NaN, needs skipNaN option?
	Nothing / no values -> Double? (null)
	(Don't convert Short/Byte to Int!)
median	T -> T? where T : Comparable<T>	For all: median of even list will cause conversion to Double	All nulls are filtered out
	Int -> Double?	and null if no elements
	Short -> Double?
	Byte -> Double?
	Long -> Double?
	Double -> Double?
	Float -> Double?
	BigInteger -> BigDecimal?
	BigDecimal -> BigDecimal?
	Number -> Double?
	Nothing / no values -> Double? (null)
std	Int -> Double	All have DDoF (Delta Degrees of Freedom) argument	All nulls are filtered out
	Short -> Double
	Byte -> Double
	Long -> Double
	Double -> Double	skipNaN option, false by default
	Float -> Double	skipNaN option, false by default
	BigInteger -> BigDecimal?	null instead of NaN in output
	BigDecimal -> BigDecimal?	null instead of NaN in output
	Number -> Double	skipNaN option, false by default
	Nothing / no values -> Double (NaN)
var (want to add?)	same as std