Fixes to callback plots

[OpheliaMiralles]

• plot_histogram: add density=True in call to np.histogram in order to get density values and not frequencies;
• plot_sample: input precipitation currently does not use the same special colormap as the target/pred precipitation sample, which is fixed in this PR
• plot_spectrum: add min_delta argument to the PlotSpectrum object to define a lower bound on the delta latitude value used to build the numpy array used for computation of power spectra. The current value is 0.0003 deg, which corresponds to 1km.

Fixes #179

[FussyDuck]

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[OpheliaMiralles]

@gabrieloks any news on this PR? I think all of the tests should pass now, I updated the code with develop today.

[gabrieloks]

@gabrieloks any news on this PR? I think all of the tests should pass now, I updated the code with develop today.

I plan to test it later today :). I also have some doubts about hard-coding 0.0003 into the code. I think this should be specified in the config files

[gabrieloks]

Looks good to me. Thanks a lot for fixing this !