Deal with unnamed db_lists

[Gabriel Alves Vieira]

Create a helper function to biblioverlap() that checks if names(db_list) is (or has any) NULL and returns a vector of numbers ( 1:length(db_list) ) to be used as set names. Else, return names(db_list).

[Gabriel Alves Vieira]

Instead, chose to add functions to check if:

1. all datasets are named
2. all column names are found in all datasets

Both raise an error with custom message if the test fails. This has been added in ae7ea72,