Copied Piecewise template from C.xml.

Generated code executes, see for example:
http://www.numericjs.com/workshop.php?link=b0ce9fbd7054e710cd4a0d82d129ecbe2cf4a03d66ff37eb7fd6aac80f71ea41
Generated from the HH 1952 model (at
http://models.cellml.org/exposure/5d116 )

But, the ODE solver built-in to numericjs is probably not "industrial
strength, certainly by comparison to the likes of CVODE.  Haven't got it
producing reasonable numerical results yet.

Still some leftover MATLAB snippets lying around.

Also, should vectorise computeAlgebraic so that algebraic variables can
also be plotted.