fig keyword allows m^2 fig.axes, if m>=n sample dims.

[fdeugenio]

Addresses #130, for cases where one sample is a subspace of the other.

Testing code, e.g.

   import corner # etc
   data = np.random.normal(size=10000).reshape(-1, 4)
   nata = np.random.normal(size=7500).reshape(-1, 3)
   fig = corner.corner(data)
   corner.corner(nata, fig=fig, color='r')

Notes

1. Does not check if samples make sense; in example above, data[:, k] is overplotted on nata[:, k] for k in {0, 1, 2}.
2. May need some cleaning of the try/except statements.