Generalize spatial types: allow any backing array type

[tkoolen]

Currently, Point3D, FreeVector3D, GeometricJacobian, MomentumMatrix, and WrenchMatrix are parameterized on their underlying array types.

On the other hand, Transform3D, SpatialInertia, Twist, SpatialAcceleration, Wrench, and Momentum are parameterized on the scalar type, and have StaticArray backing arrays hardcoded.

These types should also be parameterized on the backing array type, because:

• it enables creating mutable versions of these types
• it is required for ReverseDiff support, because of Bad interaction with StaticArrays broadcast JuliaDiff/ReverseDiff.jl#64 (comment)
• it would be more consistent with the other types