Added the ability to have species 'defaults' - thus we can now specify

Added the ability to have species 'defaults' - thus we can now specify computationDomain, divRadius, divRadiusCV, deathRadius, deathRadiusCV, babyMassFrac, babyMassFracCV, shoveFactor and shoveLimit within the tag <speciesDefaults>. Thus if the same are being used for a number of species, these now only need to be declared once in the protocol file