Splotch

* Made some adjustments to splotch.curve() by adding the following functionality:

* curve() can now take an 'orientation' parameter which sets the independent variable to either the x or y axis.
* labels are now only added to the plot calls, rather than trying to add a legend at the same time.
* Fixed bug when expression given with substitution variables, but subs not also given.
* Changed behaviour of auto-boundaries

* Fixed wrong colour map scale in hexbin when xlog/ylog=True and dens=True.
* Added missing 'sqrt' keyword in splotch.imageWCS stretch parameter.

* Fixed bug in splotch.statband() where functions could not be parsed into stat_low/high.
* Added from_mid variable that gives user control over where the statistic should be calculated from
* Removed deprecation warnings in splotch.statband()

* Fixed bug with automatic y-axis limits in plots_1d.hist()

* Renamed plots_2d.errorband() to plots_2d.statband().
* Added plots_2d.statband() equivalents with boxes and bars.
* Added plots_2d.errorbar() equivalent with bands.

* Added all plots_2d.hist2D() to plots_2d.hexbin()