Change covariate_id from big_int to varchar in sql

The type of covariate_id has been changed to varchar to account for
ICD-9 codes and other cases in which they are not actually integers.
This only affects this initial data downloading stage. The covariates
generated remain integers as before.
Update docs: README files in sql folders
Bugfix: Bug in debug logging

It was not logging the dimension sql as it was supposed to.
Add ICD-9 condition_era sql code

The previous condition_era code considered OMOP condition_concept_id
values, but this is more granular for the HDPS algorithm (as well as not
being hierarchical). The current code instead considers the equivalent
ICD-9 codes and groups on the first 3 digits. Currently the 3 digit
grouping is hard-coded, but that could presumably be changed to a
parameter.