(Hopefully) Fixed several bugs with respect to the coordinate system used...

There was a bug reported by Sven Möller that caused _action_of_Gamma0 to fail
because this method still used the old ._L list of elements in WeilModuleElement.
However, there werde quite a few bugs regarding the use of the el_index etc. - methods.
This is all due to the fact that can_coords=True has to be used
when the generators and their orders are used for the coordinate functions.
I think this should be changed to using the elementary divisors instead.
But for now, I just fixed the bugs (ugly...)

FREDRIK: Please review this. Especially: There is a line that I changed assuming that "I" should be the usual fourth root of unity contained in the complex upper half-plane. Is this correct? Did I do it correctly??
(line 1219).
Everything you did seems fine and I is indeed the usual imaginary unit as imported from sage.all.