Fixed a minor bug in PostProcessingWriter wherein if the upstroke threshold contained a point, hdf5 would fail to write the file as it cannot write format "0.4". 0.4 is now written as 0pt4

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@18449 b5f69653-2bf5-0310-8201-b0dafb8e1405