Use labels instead of variable names in validate_types()

[Bisaloo]

The idea behind safeframe in my opinion is that you only need to change the first line of your script, the make_safeframe() call when you change your data source and the new one uses new column names.

This means that all other functions should use the labels rather than the variable names.

In particular, I would expect validate_types() to do so.

In linelist for example, we know what type we expect to each tag/label but we don't know to which column this tag/label will be attached. The only way we can provide expected types to default tags is by using tags in validate_types() rather than variable names.