Refactoring pyemma.plots.markovtests.py

[cwehmeyer]

The plot_cktest() function in the current (pyemma-2.5.4) release is not flexible enough as we have seen in markovmodel/pyemma_tutorials#142.

For instance, I would like to have an auxiliary function for individual panels, like _add_ck_subplot() but with better controls (individual line/marker styles, transition labels, ...), exposed in the pyemma.plots namespace and plot_cktest() itself could also do with better controls.

Further, the entire module uses matplotlib techniques which are deprecated for scripting usage.

Issues #1229 and #1342 could be relevant here.