Grain plot widget

• Create Dataset method for computing orientation distribution and colorkey based on the dimensions (for now just with two dimensions).
• Create GrainPlot widget to show on top of the colorkey the contours of the orientation distribution reshaped as the dataset (dim1.size, dim2.size)
• Add Combobox for choosing type of map between: orientation distribution, com, fwhm, skewness, kurtosis.
• Create two plots to show com, fwhm, skewness and kurtosis for each dimension.
• Create example to easily test the GrainPlotWidget.
• Create plot for the mosaicity map.
• Add orange widget for the colorkey.