Add pointwise mapping

[fredcy]

[2017-03-06: I copied this issue, created by @folkertdev, from https://github.com/elm-community/elm-linear-algebra/issues/18 per https://github.com//issues/7. -fredcy]

As it stands, the types in this library are somewhat tedious to work with. Especially when applying
the same operation to multiple components. @fredcy mentioned on slack that conversion to List/Array is planned, but this still requires two conversions (to and from List/Array).

suggestion

Implement a pointwise map and map2 which are equivalent (in the Vec2 case) to

map : (Float -> Float) -> Vec2 -> Vec2
map f = 
    fromTuple << (\(a, b) -> (f a, f b)) << toTuple

map2 : (Float -> Float -> Float) -> Vec2 -> Vec2 -> Vec2
map2 f a b = 
    let (left, right) = (toRecord a, toRecord b) 
    in 
	fromTuple ( f left.x right.x, f left.y right.y )

edit: implementing map3 in terms of map2 isn't very practical

concrete use case:

-- maximal and minimal components of two vectors 
boundingBox a b = (map2 min a b, map2 max a b) 

points of discussion

• A logical extension would be to also introduce mapX, mapY, map...
• pointwise is an alternative name to map2 that reads nicer (but is less consistent)
• map2 is restricted to functions of type Float -> Float -> Float, where Float -> Float -> a would be more useful. Rather than adding functionality to Vector, a Tuple.Extra package could implement the various mapping functions.