[CxxRTAlib] move the getData() method of the data models in the base class

map <string,string> & EVT2b::getData() {
map<string, string>::iterator it;
for ( it = eventData.begin(); it != eventData.end(); it++ ) {
if (it->second.compare("nan") == 0 || it->second.compare("-nan") == 0 || it->second.compare("+nan") == 0)
it->second = "NULL";
}
return eventData;
}