Add global Poisson scaling option for correlated errors

git-svn-id: https://svnsrv.desy.de/basic/h1fitter/trunk@1551 b2a8d302-ca0a-427a-b7b9-081adbce06bc
Add floating point trap for division by zero for the ffpe-trap option

git-svn-id: https://svnsrv.desy.de/basic/h1fitter/trunk@1552 b2a8d302-ca0a-427a-b7b9-081adbce06bc