Issue313 integration nc data reader

See #313 (closed). Model for reader of external data implemented. NcDataReader represents an alternative to CombiTimeTable. Since an external library is used, a check especially whether the example works or not is inevitable. @mlauster, @marcusfuchs: I would suggest you for reviewer and assignee of this PR, respectively. Please suggest someone else to do the review, if you do not have time for this.