[diagnostic] implemented tests and some minor changes in data type checks
[test_data] added squeeze test
[test_data] added squeeze test
[test_data] added test for percentiles
[test_data] added test for percentiles
[test_data] added further tests for get_yearsum
[test_data] BUGFIX for interp_time routine. Handling of time was wrong as it was still based on the depreciated python standard. Now the routine is using the time conversion functions of the Data object
[test_data] tests fro num2date, date2num, save_ascii
[test_data] added test for old timeoffset
[test_data] set oldtime explicitely in test object
[test_data] further tests for time_str
[test_data2] added new file for testing data object with real sample data
[data] some doc editing