Add a convenient wrapper for GAP.Globals.Read(GapObj(joinpath(@__DIR__, "..", "gap", "exec.g"))) etc

[fingolfin]

Julia code that wishes to read specific GAP files akin to Julia's include command currently has to use code like this:

GAP.Globals.Read(GapObj(joinpath(@__DIR__, "..", "gap", "exec.g")))

That's a bit unwieldy. Perhaps we should add a macro that makes this nice, like

GAP.@include("../gap/exec.g")

(similar to how we'd do include("../gap/exec.jl") if it was a Julia file instead).

The reason why I think it needs to be a macro is that include is special in so far as that Julia creates a new method for it in each new module, which has a "pointer" to that module, and so can e.g. compute paths relative to that module's source location. Our code doesn't have that ability, unless it uses a macro.