Preserve Float type for Periodic Kernel

[molet]

Summary Tiny change to preserve float type of inputs and parameters. For most of the kernels, if the inputs and parameters have Float32 type, then the output has also Float32:

julia> v1 = 0.9f0; v2 = 0.7f0;
julia> SqExponentialKernel()(v1, v2)
0.9801987f0
julia> GammaRationalKernel(alpha=2.0f0, gamma=1.0f0)(v1, v2)
0.82644624f0

However, currently, PeriodicKernel doesn't support this:

julia> PeriodicKernel(r=[1.0f0])(v1, v2)
0.8413515018155655

The simple change of exp(-0.5d) to exp(-d / 2) in its kappa function fixes it:

julia> PeriodicKernel(r=[1.0f0])(v1, v2)
0.8413515f0

[molet]

Certainly, it's been added.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 16.29%. Comparing base (c38b366) to head (8686dd1).

Files	Patch %	Lines
src/basekernels/periodic.jl	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #560 +/- ## =========================================== + Coverage 0.42% 16.29% +15.87% =========================================== Files 52 52 Lines 1423 1430 +7 =========================================== + Hits 6 233 +227 + Misses 1417 1197 -220 ```

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[willtebbutt]

The changes that you've made do not appear to have anything to do with the test failures, so I'm going to merge.