Use metric sample for probing

Approximates tr(log(M)) using tr(log(T)) where T is the projection of M
into the krylov subspace K(M, v) where v is a sample from the metric M
(i.E. v = v_lh + v_pr where v_lh/v_pr are samples from the
likelihood/prior metric, respectively. In addition, the projected sample
is constructed by taking v_pr projecting out the subspace K(M,v) using
its eigen-basis. This ensures that both, the prior dominated part of v
and the part already covered by tr(log(T)) is projected out.

Original author: Philipp Frank
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