mulit inheritance for Dataset() class

[vergauwenthomas]

Use a multi inheritance method and a Dataset_constructor module to define the Dataset class. Here is a starting point:

from metobs_toolkit.datasetbase import DatasetBase
from metobs_toolkit.dataset_core import DatasetUserCore
from metobs_toolkit.dataset_settings_updater import DatasetSettingsCore

#Use multiple inheritance, so the full Dataset class is splid over mutliple
# child modules

class Dataset(DatasetBase, DatasetUserCore, DatasetSettingsCore()):
    def __init__(self):
        DatasetBase.__init__() #set the attributes
        DatasetUserCore().__init__()