make threejs-math dev dependency only

[andreasplesch]

Currently, many exported methods return threejs-math objects such as Euler. Often, consumers of the manifesto-3d api will have their own equivalent objects and would rather process generic javascript objects instead of having to import and rely on threejs-math (which is >400 KB). Consider switching to or adding helpers to export generic js objects and arrays (which are objects). I think all threejs-math objects have .toArray or similar methods.
This is not very critical. it was quite inconvenient to have to learn about threejs-math in order to use the library, however.

[vincentmarchetti]

Agree this is a good suggestion. Care will need to be taken, a look at Babylon.js, a peer-library to Three-js, shows that it also has Euler classes, but with some real interoperability challenges sometimes summed up as "ThreeJS is right-handed, Babylon is left-handed". Need to investigate whether quaternions representation can be specified in a more generic, but still useful, way, so we could return an array of 4 values and let each client library do its own conversion to the Euler or rotation-axis representation.