add mo_list based on flist by Jacob Williams

A generic list implementation based on flist by Jacob Williams: https://github.com/jacobwilliams/flist

It uses an unlimited polymorphic class(*) pointer variable to allow it to contain any type of data. The key can be an integer(i4), string, or any user-defined key_class.

changes to flist

• removed traverse feature (was only used to search for key)
• removed count attribute (was private and never really used)
• default integers for keys now need to be integer(i4)
• removed case_sensitive feature for character keys
• use error_message instead of error stop to be consistent in forces
• made finalizing routines impure to satisfy the intel compiler and to follow the fortran standard
• added short convenience methods add => add_clone and has => has_key
• added method add_target to add a pointer to given target to the list
• require assignment operator in key_class
• provide key_list features: list of keys by item index
• preserve list order when replacing items