[Add] issp500member: check if Date - UnID is a sp500 constituent

Signed-off-by: Oleg Komarov <o.komarov11@imperial.ac.uk>
[Change] sp500intraday: use issp500member, i.e. dump overlap, unid2permno on cusip only

Signed-off-by: Oleg Komarov <o.komarov11@imperial.ac.uk>
[Add] Count null returns

Signed-off-by: Oleg Komarov <o.komarov11@imperial.ac.uk>