PGFtoPython: add Python translations for nonExist, BIND and SOFT_BIND

Someone who is familiar with the Python translation should check this.

src/compiler/GF/Compile/PGFtoPython.hs

@@ -78,6 +78,9 @@ pySymbol (SymVar n l)    = pyDict 0 pyStr id [("var", pyTuple 0 show [n, l])]
 pySymbol (SymKS t)       = pyStr t
 pySymbol (SymKP ts alts) = pyDict 0 pyStr id [("pre", pyList 0 pySymbol ts), ("alts", pyList 0 alt2py alts)]
     where alt2py (ps,ts) = pyTuple 0 (pyList 0 pyStr) [map pySymbol ps, ts]
+pySymbol SymBIND         = pyStr "&+"
+pySymbol SymSOFT_BIND    = pyStr "&+"
+pySymbol SymNE           = pyDict 0 pyStr id [("nonExist", pyTuple 0 id [])]
 
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 -- python helpers