Change Pseudo-ECG output: include time values and allow time steps to be skipped to speed up computation.

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@18851 b5f69653-2bf5-0310-8201-b0dafb8e1405
Fix machine differences in r18851

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@18852 b5f69653-2bf5-0310-8201-b0dafb8e1405
Rename AbstractSimulationModifier to avoid duplicate file name

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@18854 b5f69653-2bf5-0310-8201-b0dafb8e1405