ajout de tsg_util/tsg_preferences.m

add missing_value field to tsg_ncvar.csv for <var>_QC variables
add autonan mode with FillValue_ attribut
correct bug on last selected display QC, tsg structure wasn't saved before display QC
add option/preference mode for autoloading data and use .mat file in prefdir to save user preferences
add tag to handle axes but don't use it