Make mean_tables and freq_tables work together better

[mbcann01]

Specifically thinking about creating Table 1 here. Look at L2C -> recreate_quarterly_report for example.

• Consistent variable naming

There will probably be other things as I dig into this.

Should we use a formula impute to the functions (i.e., similar to lm())? Would that make the distinction between response variables and grouping variables more clear?