Factorize out lemma choosing

[kristiank]

Now lemma choosing relies on document order and the lemma is chosen by number as let $lemma := ($paradigm/cell)[1].

Instead MSD features should be used:

declare function pfile:get-cell-with-msd(
  $paradigm as element(paradigm),
  $feats as map(xs:string, xs:string)
) as xs:string
{
  (: return $paradigm//cell where exists all and no more than $feats :)
}

let $attested-vars := pfile:get-attested-var-values($paradigm)
let $lemma-cell := pfile:get-cell-with-msd($paradigm, $feats)
let $lemma-wordform := pfile:get-attested-wordforms($lemma-cell, $attested-vars)