Splotch

* Added 'density' parameter tp scatter()
* Setting 'density'=True plots the colour of scatter points as their spatial number density to nearby points. This is done using a Gaussian kernel density estimate
* Added scatter example (with 'density' usage) to Examples.ipynb

* Replaced naming of parameters 'm' and 'c' in axline() with 'a' and 'b' to avoid conflict with colour parameter 'c' as per issue 38

* cornerplot() can now accepts a list of two for 'pair_type' which sets the plotting types for the lower and upper off-diagonal axes
* 'pair_kw' is now deprecated and has been replaced by explicitly 'contour_kw', 'scatter_kw' and 'hist2D_kw'
* Fixed bug in cornerplot() where labels would not be correctly assigned automatically if more than one group given
* cornerplot() now correctly shares the x/y axes of subplots according to their columns/rows without linking 1D histogram axes on the diagonal

* Added minimum number of elements per bin to plots_1d.hist(), same functionality as nmin in plots_2d.hist2D()

* Fixed bug in plots_1d.hist() with binned statistics not working for list of data
* Switched plots_2d.contourp() to use matplotlib's colourbar instead of our own

* Made modifications and bugfixes to splt.contourp(), including:

* Added the 'labels' parameter for labelling contour lines
* Added the 'smooth' parameter to apply a Gaussian smoothing kernel to the contours. 'smooth' is the sigma of the kernel
* Renamed parameter clabel --> labels
* Renamed parameter 's' --> linestyle. 's' was ambiguous and could be confused with the 's' used in scatter for "size"
* Added alliases for linestyle: 'ls' or 's' and for c: 'color'
* Fixed problem where 'clabel' applied to contour labels, rather than to colorbar as documentation stated
* 'clabel' now controls colorbar label and 'labels' controls the labels for the contours
* Fixed bug where percent, bin_type, bins, s (linstyle) would only accept lists and not other iterables
* Fixed bug where if the length of s (linestyle) was < 4, code would force linestyle='solid'
* Fixed bug when clabel[0] == None no legend would be plot in the case where other items in clabel were not None
* Fixed bug where if no ax was given and a call was made directly to ax, it would crash
* Adjusted handling of 'color' to accept: strings, floats, arrays. If color is number, now correctly samples the cmap at the correct percent value
* Adjusted usage of labels. If labels=True, automatically generate labels from 'percent'. If labels is string, display only one label in legend. If labels is an array-like, each element applies to an individual contour line. If labels = False or None, do not display a legend
* Adjusted retrieval of next colour in cycler to use: "next(ax._get_lines.prop_cycler)['color']" instead of plotting a temporary line
* Added feature where if length of 'linestyle' or 'color' were less than paercent, they would be cycled until forming a list of corrent length
* Added feature where colorbar is plot if clabel given
* Removed displaying of labels as a default behaiviour
* Removed automatic setting of 'linestyle' to 'solid' if col was not string
* Removed multiple calls to "output = Params. ..."
* Removed "if None in (percent, cmap, clim, s, ...)" as this conflicted with numpy arrays
* Removed 'viridis' default from 'cmap' docstring