Splotch

* Fixed following problems with splotch.curve_piecewise

* Not able to plot horizontal/vertical lines
* Error is var not explicitly given.
* expr and intervals does not accept tuples
* subs had to be specified for all expressions rather than those they were relevant to.

* 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.