Add "previous" User PD function

[sfreemire]

For IPv6 User PD:
- Increment with "next" as usual.
-Decrement by recreating iterator object and calling it (current count -1) times.

 ```
  # ...
  v6upd_obj = next(self.v6upd_gen)  # Initial User PD creation
  self.updcount = 0
  # ...

  # Increment / decrement User PD 
  match pdchg:
     case 'next':   # <-- for incrementing
        if self.psidstr == self.pslen * '1':   # if all ones:
           self.psdistr = self.pslen * '0'.    # roll over to all zeroes
        else:
           self.psidstr = bin(int(self.psidstr, 2) + 1)[2:].zfill(self.pslen)

        v6upd_obj = next(self.v6upd_gen)       # get "next" from upd generator object
        v6upd_netadr = v6upd_obj.network_address
        v6updbin = f'{v6upd_netadr:b}'
        v6upd_adr = v6upd_obj.with_prefixlen
        self.updcount += 1
           # print(f'Maximum reached, PSID starts over: {self.psidstr}')  # test message

     case 'previous':   # for decrementing
        if self.updcount > 0:  # <-- if '1' in self.psidstr (but not all '1's):
           self.psidstr = bin(int(self.psidstr, 2) -1)[2:].zfill(self.pslen)
           self.v6upd_gen = self.bmrv6p_obj.subnets(new_prefix = self.v6updlen) # <-- re-create upd generator

           for _ in range(self.updcount):  # <-- range should be 1 less than current count (range starts at 0)
              v6upd_obj = next(self.v6upd_gen)  # <-- call "next" upd (updcount - 1) times
           self.updcount -= 1

           v6upd_netadr = v6upd_obj.network_address
           v6updbin = f'{v6upd_netadr:b}'
           v6upd_adr = v6upd_obj.with_prefixlen
        else:
           # print(f'Minimum reached, no change, self.psidstr is: {self.psidstr}')  # test message

     case _:
        pass    # Not needed? Consider error message