Unused methods

[gerritholl]

Some methods are apparently unused in fogpy. Apart from plotting-related functionality which I have suspended, these include:

• algorithms.DayFogLowStratusAlgorithm.get_lowcloud_cth
• algorithms.PanSharpeningAlgorithm.get_dist_threshold
• algorithms.PanSharpeningAlgorithm.get_sza_in_range
• algorithms.PanSharpeningAlgorithm.get_bt_dist
• algorithms.PanSharpeningAlgorithm.get_turning_points
• algorithms.PanSharpeningAlgorithm.get_slope
• filters.CloudFilter.get_slope_decline (used by plotting routines only?)
• SpatialCloudTopHeightFilter_old.filter_function
• lowwatercloud.LowWaterCloud.get_vapour_pressure