removed to_frame method until a more general method is found
added "dims" attribute to Dataset, removed "dimensions" for consistency
with DimArray.
fixed Dataset's attributes I/O
Dataset is now a subclass of OrderedDict
dataset repr improved
now can append netCDF variable to an existing file
additionally, NCDataset and NCVariable classes for access similar to
DimArray
removed NCDataset object which does not bring much
read_nc and write_nc for Dataset