Change default in covar_to_nui for stat. errors to scale following poisson

git-svn-id: https://svnsrv.desy.de/basic/h1fitter/trunk@1546 b2a8d302-ca0a-427a-b7b9-081adbce06bc
Fill ALPHA array in a central place, fix missing uncor_const piece for it.

git-svn-id: https://svnsrv.desy.de/basic/h1fitter/trunk@1547 b2a8d302-ca0a-427a-b7b9-081adbce06bc