[Change/Add] mapUnid2mst: factor out uniqueID routine

Signed-off-by: Oleg Komarov <o.komarov11@imperial.ac.uk>
[Change] sampledates: nanfills before filtering for refdates

Signed-off-by: Oleg Komarov <o.komarov11@imperial.ac.uk>
[Add] getBetas: retrieve/prepare betas and applying some filters

Signed-off-by: Oleg Komarov <o.komarov11@imperial.ac.uk>
[Change] sample dates inside getMktCap

Signed-off-by: Oleg Komarov <o.komarov11@imperial.ac.uk>
[Add] Mom vs Mom on spy vs Mom on proxy #1

Signed-off-by: Oleg Komarov <o.komarov11@imperial.ac.uk>
[Fix] sampledates: correct fix for out of boundary

Signed-off-by: Oleg Komarov <o.komarov11@imperial.ac.uk>
[Fix] getMktCap: make sure null shares are NaN

Signed-off-by: Oleg Komarov <o.komarov11@imperial.ac.uk>
[Add] Cross-sectional Beta average sorted by MktCap

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