[make] check2.7 -> check2
[r_value] Drop support for covariance

The R value only needs variances, for D large, the difference between
covariance matrix ~D^2 and variance ~D is significant.

o set default critical R value to 2
o remove `multivariate_r`
o update doc strings
o update example `r_value`
[linalg] Add chol_inv_det

Use direct access to lapack via scipy to compute the inverse and
log(det) via the Cholesky decomposition. This is both faster and
more robust than np.inv and np.det, which work on non-symmetric
matrices, too.
[density, mix_adapt] Use chol_inv_det in LocalGauss
[variational] Use chol_inv_det

Some more members to compute inv and det only once. Det now computed on
the log scale and much more robust against overflows

o output 15 digits on bound
o change interface of Wishart*