Non-isotropic length scales for graph-based Gaussian processes

[tillahoffmann]

This PR allows graph-based Gaussian processes to have different length scales along different dimensions of the covariate space.

[tillahoffmann]

On hold until fix from stan-dev/math#3084 is released.

[skiptoniam]

@tillahoffmann has this been updated? for the stan libs?

[tillahoffmann]

Quick update: it looks like the new release date for the bugfix we need is November 11 (see here for details).

[tillahoffmann]

It looks like the next release date has been pushed back to late November/early Decembers so we'll have to wait a little longer to merge this feature.

[tillahoffmann]

We'll have to weight for the next release candidate because the _jacobian suffix for functions is no longer allowed by accident (see here for details).

[tillahoffmann]

@skiptoniam, thanks for the patience with this PR. I'll release a new Python package shortly and also port the changes over to the R package. I'll post another update here once that's done.

[tillahoffmann]

This has now been published on PyPI as gptools-stan==0.2.1 and CRAN as gptoolsStan==0.2.0.