Constants not handled good (and "swap"-logic)

[shukon]

  File "/home/shuki/Repos/fanova/fanova/visualizer.py", line 416, in create_most_important_pairwise_marginal_plots
    self.plot_pairwise_marginal((param1, param2), show=False, three_d=three_d, resolution=resolution)
  File "/home/shuki/Repos/fanova/fanova/visualizer.py", line 171, in plot_pairwise_marginal
    choices, zz = self.generate_pairwise_marginal(param_indices, resolution)
  File "/home/shuki/Repos/fanova/fanova/visualizer.py", line 121, in generate_pairwise_marginal
    swap = len(grid_fanova[1] > len(grid_fanova[0]))
TypeError: object of type 'numpy.int64' has no len()

1. Constants cause a TypeError because their np.arrays in the grid-logic are np.squeeze()ed away, so they have no length and throw errors.
2. While I'm not 100% sure what the swap in axes is supposed to do, there clearly is a logical mistake with the brackets here. Since it does not seem to have caused errors, it's unlikely to be critital, still should be fixed