`QuadFormAndIsom`: Serialization infinite order isometries and a minor fix

[StevellM]

Up to now, since there is no serialization of PosInf, we could not store the order for isometries of infinite order. Since, in the finite or trivial case, this order n is at least -1, for the storing I have opted for n = -2 in the case of infinite order.

Eventually, even if we implement the serialization of infinity, I guess it could stay as is: there is no possible conflict, and it is something that humans are not suppose to read.

@antonydellavecchia does it seem good ? Any objection ?

The extra minor fix is trivial: OqAinOD, by construction, sends the identity to the identity (similarly for the other one).