Add option for vectors and tuples

[JakobAsslaender]

Hi, this is a minor change adding the option to call DiagOp with a vector of operators or an NTuple of Operators. My goal is to prevent having to use the splat operator to improve speed.

The change should be non-breaking.

@tknopp: If approved, would you mind taking care of the release, as I still don't know how to release this multi-package system of MRIReco.jl

Thank you!!

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (77dac78) 86.68% compared to head (281cf80) 86.68%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #172 +/- ## ======================================= Coverage 86.68% 86.68% ======================================= Files 10 10 Lines 368 368 ======================================= Hits 319 319 Misses 49 49 ```

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[JakobAsslaender]

@tknopp, a friendly ping.

Can you approve, merge, and release?

[tknopp]

done, release it triggered!

[JakobAsslaender]

Thank you!

I just realized that LinearOperators.jl has similar functionality: https://github.com/JuliaSmoothOptimizers/LinearOperators.jl/blob/7a5456fb5d9c39c3ce29bc212e5f3c18943e6172/src/special-operators.jl#L241

Any reason to duplicate that work–or shall we decrepate DiagOp? I see that their mul! operation is not parallelized, but that could maybe be changed with a PR.