* fix bug in the simplified parser where repeated keywords were not emitted properly,
  again #32 but slightly different. This reintroduced the usage of tuples for
  multi-word keywords as a way to do repeated keyword and multi-word merging.
  Mostly visible internally since JSON and YAML do not emit tuples.
* introduce switch to select the string to which a repeated default keyword is mapped to,
  as it seems, aiida-cp2k chose `" "` (one space), as requested in #36