Vector multiplications

[Licini]

Triggered by discussion of #1344 and #1248

I'm more on the side of allowing __rmul__, since a lot of people have the habit of writing equations in such order. On implementation level it would be also tricky to show a warning to say that we don't allow it (in that case __rmul__ is anyway needed).

Also adding support for element-wise multiplication and division for vector * vector, aligning with other libraries like Numpy, Tensorflow, Pytorch, R ... (In MATLAB this returns dot product tho)

The PR is created more for discussion purpose, let me know what you guys think

What type of change is this?

• [ ] Bug fix in a backwards-compatible manner.
• [x] New feature in a backwards-compatible manner.
• [ ] Breaking change: bug fix or new feature that involve incompatible API changes.
• [ ] Other (e.g. doc update, configuration, etc)

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