Allow for more flexibility in compute_variables function

[alastairmatheson]

Some ideas:

1. Allow for a list of variables that the dataset includes (e.g., SSN might be optional) and then only run sections for selected variables.
2. Allow users to map variables names rather than assume they are set up as fn1, ln1, etc. I don't feel super strongly about this one; users have to do some cleaning anyway so name standardization could just be part of that.