Add NAN code support to claims_hosp

[dshemetov]

Description

Work as part #838.

Changelog

• Now instead of the include flag not writing a CSV file, a CSV file is still written but with all the columns NAN
• Adds missingness code columns; writes a privacy code for se and sample_size by default, writes a data insufficient code if the value doesn't have the include flag
• Should naturally handle divide by zero cases in the daily rate calculation by marking them with insufficient data.
• Source nans were already zero-filled on data load (not changed in this PR). A different imputation may be worth trying if we have issues in the future.

Fixes

• Partially addresses Update indicators with NAN codes #838