Latvian: finally handles PassV2 clauses in a more or less adequate way

Extends the previously introduced support for specifying the verb valence (verb-dependent subject and object cases), involving functions up to PredVP and RelVP.

In Latvian, the passive voice is not used if the agent (subject) is known; to preserve the information structure (i.e. the word order), a clause like 'A is <done> by B' is linearized in the active voice ('A <does> B') where A has the object case (e.g. Acc), and B - the subject case (e.g. Nom). Thus, the verb valence patterns are swapped on-the-fly.

This is still a rather quick & dirty implementation: parameters and linearization types have to be optimized (VerbLav), the use of PassV2 in AdvVP is problematic as it doesn't apply VPSlashPrep / ComplSlash, etc.

lib/src/latvian/AdverbLav.gf

@@ -18,7 +18,7 @@ lin
   ComparAdvAdjS cadv a s = { s = cadv.s ++ a.s ! (AAdv cadv.d) ++ cadv.p ++ s.s } ;
 
   -- FIXME: postpozīcijas prievārdi
-  PrepNP prep np = { s = prep.s ++ np.s ! (prep.c ! (fromAgr np.a).n) } ;
+  PrepNP prep np = { s = prep.s ++ np.s ! (prep.c ! (fromAgr np.a).num) } ;
 
   AdAdv = cc2 ;
 
@@ -27,7 +27,7 @@ lin
   AdnCAdv cadv = {
     s = case cadv.d of {
       Posit => cadv.s ++ cadv.p ;
-      _ => NON_EXISTENT
+      _     => NON_EXISTENT
     }
   } ;