Splotch

* Modified splotch.curve() so that an independent variable is not strictly required, allowing for constant expressions, i.e. horizontal lines.

* Added function curve_piecewise() which works similarly to curve() but can be parsed piecewise expressions.

* Upgraded splotch.axline() so that it can accept a list of axes on which to plot lines.

* Fixed spelling errors

* Fixed bug in splotch.subplots() that allowed any string to be parsed into parameters va and ha, with no warning or exception.

* 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