Print about xgap package when loading OSCAR

[joschmitt]

When I do using Oscar I get

#I  You may wish to install the xgap package
#I  and enjoy the graphic capabilities of SONATA.

after the copyright message.

[fingolfin]

Thanks for filing the issue, @joschmitt !

Thanks to @ThomasBreuer for tracking it down; turns out it is a problem in Oscar (and perhaps GAP)