[WIP] Use scalar parameters with ParameterHandling

[devmotion]

This PR implements https://github.com/JuliaGaussianProcesses/KernelFunctions.jl/issues/299#issuecomment-943761371. See the comment and the additional explanations in the issue for more details.

This PR is still WIP and tests will fail (I assume).

[devmotion]

By using only we could get rid of a lot of the length(v) = = 1

I know, and initially I used it in some places :slightly_smiling_face: However, it requires Julia >= 1.4 and hence does not work with Julia 1.3.

[theogf]

Since this is going to a be a (massively) breaking release, maybe we can drop support < 1.6

[devmotion]

I would be fine with this but I didn't want to make this change without prior discussion. Also I assume Julia 1.6 might be the LTS before this PR is ready and merged :slightly_smiling_face:

[codecov[bot]]

Codecov Report

Attention: 361 lines in your changes are missing coverage. Please review.

Files	Coverage Δ
src/KernelFunctions.jl	100.00% <ø> (ø)
src/basekernels/cosine.jl	0.00% <ø> (-100.00%)	:arrow_down:
src/basekernels/exponentiated.jl	0.00% <ø> (-80.00%)	:arrow_down:
src/basekernels/nn.jl	0.00% <ø> (-98.00%)	:arrow_down:
src/basekernels/piecewisepolynomial.jl	0.00% <ø> (-89.48%)	:arrow_down:
src/basekernels/wiener.jl	0.00% <ø> (-92.86%)	:arrow_down:
src/transform/transform.jl	0.00% <ø> (-54.55%)	:arrow_down:
src/TestUtils.jl	0.00% <0.00%> (-93.25%)	:arrow_down:
src/transform/ardtransform.jl	0.00% <0.00%> (-26.67%)	:arrow_down:
src/utils.jl	4.59% <40.00%> (-87.07%)	:arrow_down:
... and 23 more

... and 21 files with indirect coverage changes

:loudspeaker: Thoughts on this report? Let us know!