Improve lower_vars workflow (created May 1, 2020 by @dtburk on mnpopcenter/ipumsr)

[dtburk]

May 1, 2020 @dtburk:

In response to mnpopcenter/ipumsr#56, we added a warning message when the lower_vars argument to any of the read_ipums_* functions is ignored. As described in the discussion of that issue, the reason the argument is sometimes ignored is to make sure the case of the variable names stays in sync between the data and the ipums_ddi object associated with the data. Keeping these in sync is helpful if the user wants to use a function like set_ipums_var_attributes() that attaches metadata from the ipums_ddi to variables in a data.frame. However, by making these metadata-attaching functions a little smarter, we can probably allow the case of variable names to get out of sync between ipums_ddi and data.frame, while still allowing users to attach metadata if they want to. Once we make those fixes, we can allow users to convert variable names to lowercase when they read in the data, even if they have already read in the DDI.