[datacube_util.Dataset] several changes to support the following:
[datacube_util.Dataset] several changes to support the following:

- pass a list of files upon initialization
- pass the measurement units upon initialization
- allow combination of datasets via + operator and sum function
- set units of all measurements in the dataset
- get measurement parameters as dictionary

johntruckenbrodt/pyroSAR,johntruckenbrodt/pyroSAR