remove `BaseNumberFieldExtra`

[edgarcosta]

@SamSchiavone and I do not understand the purpose of BaseNumberFieldExtra.
It is not used elsewhere in the package except in the examples

see:

@SamSchiavone said:

Yeah, I don't understand the purpose of BaseNumberFieldExtra. It doesn't seem to be used elsewhere in the package. The only real difference I see between the two are here

Lrel<r> := NumberField(f); L := AbsoluteField(Lrel);
L`base := L; L`base_gen := L.1; L`CC := ComplexFieldExtra(prec); L`iota := InfinitePlacesExtra(L)[1];
hKL := hom< K -> L | >;

vs here

Lrel<r> := NumberField(f); L := AbsoluteField(Lrel);
L`base := K`base; L`base_gen := L ! Lrel ! K`base_gen; L`CC := K`CC;

when assigning L`base.

[edgarcosta]

@JRSijsling says something along the lines regarding BaseNumberFieldExtra:

Probably, it has something to do with relative extensions, so say a quadratic over a quadratic.
It probably does not matter that much if you have a curve over a number field.