SIC - looking at missing n1 dn file, etc.

Removed HIV from list in step 7a code, initially checking criteria
Fixed counts in comments in section getting n1, etc interviews to
reflect serious medical illness criteria excluding HIV
Added variables looking at age, timing of n1 interview
For identifying those that meet the criteria the first interview where
they are in our sample, added rows checking why so many obs are missing
the denominator data 1 year pre the n1 interview
Modified table checking outcomes on each part of the criteria to remove
nh only and hospitalization only categories since these aren't
meaningful groups for our purposes
Started OOP codebook