Helper function for multiplication of two quaternions.
Concatenation of two rotations R1 followed by R2

## Usage

`product_Q4(q1, q2, normalize = FALSE)`

## Arguments

- q1, q2
two objects of class `"quaternion"`

. first rotation R1 expressed by q1 followed by second rotation R2 expressed by q2

- normalize
logical. Whether a quaternion normalization should be applied (TRUE) or not (FALSE, the default).

## Value

object of class `"quaternion"`

## Note

Multiplication is not commutative.