edpsciences_package: conversion improvements

* Keeps the abstract formated in chapters.

* Seperates the author keywords in different fields.

Signed-off-by: Georgios Papoutsakis <georgios.papoutsakis@cern.ch>
global: subscripts superscripts support

Signed-off-by: Georgios Papoutsakis <georgios.papoutsakis@cern.ch>