Complete the abstract group chapter

[denisrosset]

The following items are missing:

• Description of the elements of an abstract group, i.e. strings that represent words in the generators
• How strings that are not equal as string can be equal as group elements
• How abstract groups are used for group recognition results
• How to compute the abstract group corresponding to a permutation group
• How to compute a permutation representation for an abstract
• How to add an abstract group to RepLAB's atlas