Fix IndependentPosteriorMultiOutput for kernels with active_dims

[BLarvaron]

I propose to replace use of K method by call as explained in #1723

PR type: enhancement

Related issue(s)/PRs: #1723

[st--]

Hi @BL7X4 ! thanks for catching this, I suspect you are right. You need to run make format (or manually call the black formatter) to get the format-checker to pass (and merge in develop).

It'd be great if you could also add a test case that confirms that having kernels with active_dims was broken before, and is fixed by this change!

[st--]

(BTW- it's generally helpful to give PRs more descriptive names, for example "Fix IndependentPosteriorMultiOutput for kernels with active_dims" in this case - to make it more immediately obvious what it is about and here that it's fixing a bug:))

[st--]

@BL7X4 to merge this in, you would need to:

1. merge the current state of the master branch back into your fork/branch
2. make sure you run the black formatter before committing
3. add a small test that demonstrates the bug (without your change) and that this change fixes it (with your change).
   Thanks for your contribution!