Update County at DX Analysis algorithm

[depryf]

Current logic:

if DX year priro to 2000, use 1907/80/90 census fields
else if DX year prior to 2010, use 2000 census fields
else use 2010 census fields.

New logic:

if DX year priro to 2000, use 1907/80/90 census fields
else if DX year prior to 2010, use 2000 census fields
else if DX year prior to 2020, use 2010 census fields
else use 2020 census fields.