feat: Shorten Markov chain if initial matrices bad
feat: Shorten Markov chain if initial matrices bad

Automatically remove bad transition matrices at the beginning of
the Markov chain unless they can be replaced with interpolated
versions.

This feature addresses an issue raised with commit 02d210a and
related to the fact that the matrix interpolation algorithm breaks
if the chain begins with badly specified transition matrices.

Now, even such matrices are automatically scrapped, and a shortened
chain returned. The algorithm should now be robust enough to handle
most special cases.

Resolves: #5

federicomariamassari/willow-tree