Add constructive recognition for SL_n

[fingolfin]

Sometimes this still runs into an error:

gap> res:=RECOG.FindStdGensSmallerMatrices_SL(SL(50,5)); time;
Error, List Element: <list>[2] must have an assigned value in
  for ele in eigenspaceGenerators[2] do
    vec := zerovec;
    for j in [ 1 .. Size( ele ) ] do
        vec := vec + ele[j] * result[j];
    od;
    Add( sl2eigenspacebase, vec );
od; at GAPROOT/pkg/recog/gap/projective/constructive_recognition/SL/GoingDown.gi:605 called from
RECOG.Computesl2Subspace( baselist, eigenspacelist
 ) at GAPROOT/pkg/recog/gap/projective/constructive_recognition/SL/GoingDown.gi:545 called from
RECOG.SLn_constructsl2WithSmallerMatrices( sld, d, q
 ) at GAPROOT/pkg/recog/gap/projective/constructive_recognition/SL/main.gi:162 called from
<function "unknown">( <arguments> )
 called from read-eval loop at *stdin*:2
type 'quit;' to quit to outer loop

[fingolfin]

The TestRecogGL tests systematically fail.