Add option to use either Magmas standard forms or GAPs standard forms

[ssiccha]

I think if we hade to choose, then we might only want support constructing classical groups which respect GAPs standard forms. However, if it is possible without too much hassle, then we should also add the ability to construct classical groups which respect Magma's standard forms.

[fingolfin]

... or any form? In fact where is the hassle, it's a simple matter of conjugation?