no progress bar if tree is empty
optional quiet mode (ignore root_numpy warnings)
Merge pull request #163 from ndawe/master

no progress bar if tree is empty
create empty tables for empty trees
Merge pull request #164 from ndawe/master

create empty tables for empty trees
add optional userfunc in root2hdf5 to perform user-defined processing of input tree before converting to PyTables
Merge pull request #165 from ndawe/master

root2hdf5: optional user-defined function to process input tree before conversion to PyTables