This function loads a .mat file containing a structure and appends all the loaded fields and respective values to an older structure.
Any new fields are simply added.
This is very useful when working with preferences files, where you may create a structure with the default values and then load the user preferences from previously saved file.
If for instance, the user file is older and doesn't contain newer fields, the default ones with fill the spots.
PREFS_NEW = LOAD_PREF_FILE(PREFS_DEFAULT, PREFS_FILE) , PREFS_DEFAULT and PREFS_NEW are structure type. PREFS_FILE is a string containing the file name to be loaded.