Why L1 for stationary kernels by default?

[mvsoom]

Hi Dan,

I really like this library; thank you for designing it.
I'm working on extending it to specialized algebra for spectral kernels.

My question: in stationary.py, why is the default Distance for Stationary kernels L1Distance?

This has the consequence that eg. Matern32 kernel with default instantiation has non-standard behavior whenever the input dimension is larger than 1.
I'm not sure if many users would suspect this behavior, even though it is documented.

Cheers,
Marnix