Specifying Species By Number Per MM3 rather than a set number

Changes to initial setup of species such that these can be declared in terms of volume rather than as a number of cells. Now the user has the option to specify the number of cells per millimetre cubed. This is then
used to calculate the number of cells that should be introduced to the simulation initially. However the update goes a little further than this. In all the examples, the cell initial setup site is the whole length of the Y and Z domains, with a height of 1 resolution (to ensure the cells are initially near the substratum). Now the user will have the opportunity of not declaring an initial site at all: if they do this, iDynoMiCS assumes the area to be the length of the Y and Z domains, with a height of 1 resolution. This detail has been specified in detail, with examples, in the new example protocol file that is included in this commit.