Function: nfmodprinit
Section: number_fields
C-Name: nfmodprinit
Prototype: GG
Help: nfmodprinit(nf,pr): transform the prime ideal pr into modpr format
 necessary for all operations mod pr in the number field nf.
Doc: transforms the prime ideal \var{pr} into \tet{modpr} format necessary
 for all operations modulo \var{pr} in the number field \var{nf}.
 The functions \tet{nfmodpr} and \tet{nfmodprlift} allow to project
 to and lift from the residue field.