Function: bnfdecodemodule
Section: number_fields
C-Name: decodemodule
Prototype: GG
Help: bnfdecodemodule(nf,m): given a coded module m as in bnrdisclist,
 gives the true module.
Doc: if $m$ is a module as output in the
 first component of an extension given by \kbd{bnrdisclist}, outputs the
 true module.
