What to do with LoKi specific YAML?

The following discussion from !885 (merged) should be addressed:

• @graven started a discussion: (+1 comment)

  For a separate MR: there is still one case where add_filter is used to create a LoKi_HDRFilter, and that is because it is currently possible to specify eg HLT_PASS('SomeLine') in the yaml file (using the evt_pre_filters field), which is then used to call add_filter. This type of functionality cannot be implemented 'as is' using ThOr (i.e. using create_lines_filter), as the yaml field implies plain strings and not python objects imported from Functors.

  Probably the best migration path is to add a distinct field in the yaml file which just lists the required lines (as text), and use that as argument to create_lines_filter (after splitting the string into a list of lines), and at the same time flag the use to the current evt_pre_filters (which explicitly hardwires the use of LoKi already in the yaml file) as deprecated -- but this is left for a separate task.