Improve dot product

[cshaa]

• makes dot product antilinear in the first argument (fixes Complex dot product doesn't conjugate #1761)
• makes dot product more robust
  • it originally accepted only 1-dim vectors and arrays (see this comment)
  • now it also accepts column vectors as either matrix or array
  • types can be mixed
• adds implementation for sparse vectors
• makes multiply use dot for inner product
  • removes duplication in codebase

#coronathlon

[josdejong]

Thanks Michal, I like where this is going! It will be a breaking change so let's publish it with a new major version number.

It may be good to add a few unit tests to multiply to test the behaviour with complex numbers, since that is changed in your improved version implementation of dot?

I see you added a TODO "unify implementation with dot(a, b)", but it looks like you already did that, right?

[josdejong]

Michal I've merged your improvements in dot product in a new v7 branch. See also #1532. Thanks again!

Will await a couple of days before publishing v7 containing just this change (which is fine, but let's give some time to think about other possible changes we want to have in now).