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