90 Refine calibration constant selection.

[HavardStridBuholdt]

Addressing issue #90

Also added default calibration constants.

Had to redefine how we store the calibration constants. The structure of data_cube.pol_cali and data_cube.LC are still the same but they now include and additional entry default that stores the default constants per channel. data_cube.etaused and data_cube.LCused are now nested dictionaries with entries {'eta'/'LC': ... , 'eta_std'/'LCStd': ..., 'method': ...} per channel. The method entry shows where the chosen calibration constant comes from, D90, D90_db, klett, raman, klett_db, raman_db, or default.

Default constants are only selected if no other exists.

The update also introduces the following new config variable dependencies:

• flagDepolCali: enable or disable the retrieval of DCs from the measurement.
• flagLCCalibration: enable or disable the retrieval of LCs from the measurement.
• flagUsePreviousDepolCali: enable or disable the loading of DCs from the database.
• flagUsePreviousLC: enable or disable the loading of LCs from the database.
• calibrationDB: now used to construct a default database-path if the loading of CCs is enabled but no path was given in the function input.

If both the retrieval of CCs from the measurement and the loading of CCs form the database is disabled, the default values will always be chosen.

[HavardStridBuholdt]

Merge #87 before this one.