added `labels` attribute and parameters
added _mean, _std and _var for checking purposes
can now pass variable argument list to group (as well as tuple)
fix a bug in dataset.py
now can use variable list for reshape, transpose and group
pandas exchange improved
fixed bug in regional_mean
regional transform nans
delitem method to dataset.py
workaround bug fix: to_dataset works fine, but need to check what can be
wrong in align_Axes