Simplify variable reference table

[nevrome]

In my quest to cut away everything none-essential from c14bazAAR I would like to simplify the variable reference table. The following steps seems sensible:

1. So far the list stores a lot of additional variables the databases provide, but are not used by c14bazAAR. I would like to remove these. It's not really useful information, given the extreme heterogeneity of variables and variable meanings across databases.
2. With this change the data structure can be reorganized once again: Columns for each variable in c14bazAAR and rows for each database and their relevant variable equivalents.