slight improvements to IndexKernel for the rank == 0 (diagonal) case

[rmgarnett]

This PR tweaks IndexKernel to have slightly better performance/behavior in the corner case of rank == 0, where the underlying covariance matrix is diagonal. Although this case is atypical from the point of view of multitask GPs, I have found this construction useful as a component of larger covariance structures.

Two changes are proposed:

• the kernel only registers the covar_factor parameter (the low-rank component when rank > 0) when rank > 0
• in the rank == 0 case, the covar_matrix() method returns a DiagLinearOpeator rather than a PsdSumLinearOperator(RootLinearOperator, DiagLinearOperator)

[rmgarnett]

I've added a branch to _eval_covar_matrix() to handle the diagonal case to partially address @Balandat's comments.

[Balandat]

I guess I would still like to see this covered by a unit test though :)