throw runtime error rather than assertion
OpImageReader: provide min,max value as drange meta parameter
Revert estimating drange in OpImageReader

After discussions with Christoph, this does not seem to be
the right place to collect such data. Changing the meta dict
will always trigger a full reconnect of the graph, and will
also set the data of the ouput slot dirty.
Merge branch 'master' of ssh://github.com/ilastik/lazyflow
wip