106 commonly used ses variables

[shijiaSMH]

Closes issue

106

Major changes:

• Add a function to merge all commonly used socioeconomic (SES) variables to a cohort input.
• Add a vignette to provide user basic understanding about variable concepts & how to use

Additional information, if applicable:

For reviewer, can you pls review function (nbhd_ses_variables.R) & vignette (nbhd_ses_variables.Rmd) for clarity?

• For vignette, it's a good sign that you feel confident using all variables after reading. Can you ensure all reference links work properly?
• For function, no concerns so far.

Saved vignette output here: R:\GEMINI\Research Projects\Rgemini\nbhd_ses_variables

I didn't do unit test since function is about merging.

Use case: Expect analyst to use nbhd_ses_variables.R function to grab all the SES variables needed, and use nbhd_ses_variables.Rmd as a reference to obtain basic understanding of those variables.