Rewrite Abelian Groups

Comment from John Cremona:

If I were there (sorry for the millionth time) I would want to contribute to 
both the projects you have listed!  Especially the second one (abelian groups).  
I have been looking at #5882 with interest and expected that a rewrite of 
abelian groups would follow.

One plea: make sure that both additive and multiplicative notations
are supported.  It's really only a matter of input & output, plus some
fairly obvious terminology changes (e.g. you allow A(0) only when A is
additive and A(1) only if it is multiplicative), all the underlying
computation is in common (and indeed is handled by #5882).  For
example, in the now defunct ticket #4739 I did add additive versions
of the existing abelian group code (which uses multiplicative
notations exclusively despite being based on an additive
implementation) as I got fed up with elliptic curve torsion groups
being displayed multiplcatively.

The way I did this at #4739 might have some useful code which can be
recycled.  It was based on 3.2.1 six months ago and obviously does not
now apply cleanly.

[I could have added the above comments to the wiki page but was not
sure of that was appropriate.  Feel free to do so if it is.]