Spiral Wave Input Files, Pacing protocols definable in Input File

With the new PiecewiseParsedFunction, pacing protocols can be defined in the input file instead of having to hardcode them. The ElectrocardioForcing kernel now expects such a (or a different) function as pacing definition.
Current status of the C-Electrocardio2D.i input file corresponds to the "almost complete spiral wave" as presented in the CCMC meting of December 2.