added a clean test_read function to test reading an example netCDF file with read_nc
rearrange reading netCDF test as a class, testing for both read_nc and
open_nc forms
add labels attribute to dataset, for consistency with DimArray
summary() : now print information to screen
see summary_repr to have the equivalent string representation
test_nc.py : new indexing test on reading for VariableOnDisk class +
bugfixes