2D: use hv.TriMesh instead of hv.Contours

# new (holoviews 1.10)
%timeit learner.plot(tri_alpha=0.1)
11.7 ms ± 803 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)

# old (holoviews 1.9*)
%timeit learner.plot(tri_alpha=0.1)
317 ms ± 17.3 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)

nuff' said

Result:

closes #36 (closed)