Find the pH value which is the centre of mass of the pH limitation function (used for the pH trait)
pHcentreOfMass(strainName, groupName, pHLimFunc, parms)
strainName
: Name of the straingroupName
: Name of microbial grouppHLimFunc
: function specified in rateFuncs$pHLimFuncparms
: List of all parameterspH value at centre of mass