Changes

Next we need to determine precedence if a PortCo has both an IPO and an M&A record. There are 173 such records. This is done in the query '''ExitKeys'''. PortCoExit (49136) is then built. This version fixes an error from before, where announceddate and not effectivedate was being used as exitdate for M&As. Note that exitvaluem is left as null for non-exits. PortCoAliveDead still uses a 5 year interval, but the date truncation date is updated to 9/22/2019. exitvaluem is then updated to 0 for those that are deemed dead without exit and left as null for non-exits. Exitdate and year are left as is, as new fields (deaddate, deadyear) are created appropriately.