adding conversion functionality

[huckl3b3rry87]

I am wondering if it would be appropriate to add some conversion functionality to this package, for instance converting from Quaternions to Euler angles and vice versa. For now, maybe we can just import the functionality from C or python modules such as these

[hyrodium]

Rotations.jl supports the features such as conversion from quaternion to Euler's angles. (JuliaGeometry/Rotations.jl#175)

Note that this package Quaternions mainly focuses on algebraic properties.