implement Vector operators __add__, __sub__, __mul__

[justbuchanan]

No description provided.

[adam-urbanczyk]

Looks good. One question though: do you have a specific use case for the Vector object in mind?

Also it would be nice to add some tests.

[justbuchanan]

cqparts uses Vectors fairly extensively for moving between coordinate systems, among other things. Since several of the cadquery APIs accept and return Vector objects, it works out well to use them throughout. It would certainly be possible to use methods like multiply and add directly, but the operators are really convenient.

I added some test cases and implemented __truediv__ to complete the basic four operators.

[adam-urbanczyk]

Thanks for the contribution @justbuchanan