[with patch] refactoring to improve finite field reference manual

[malb]

The patch removes FiniteField_prime_modn from finite_field.py because it was odd that this implementation was the only showing up in the reference manual. Also, GF is now defined in rings.all rather than in rings.finite_field to avoid that the documentation for it shows up twice. Finally, a more verbose description of the finite field module is given at the top of the finite_field.py file and some doctests were added to FiniteField_prime_modn.