Clarify documentation of projective nodes

[TWiedemann]

1. The information whether a RecogNode ri is projective or not is currently stored in the boolean ri!.projective, but this is not documented anywhere. Maybe there should be a filter IsProjectiveRecogNode or something like that, but at least it should be documented.
2. The documentation of various attributes is misleading for projective nodes ri. For example, Grp(ri) is not the group represented by ri but the group whose projective quotient is the group represented by ri, and this is not clear from the documentation. I think the documentation of Homom and NiceGens should also clarify that they are defined on/in Grp(ri) and not necessarily on/in the group associated to ri.

[TWiedemann]

Problem 2. above is even worse than I thought. If a node ri is projective, then the group associated to ri is not necessarily the quotient of Grp(ri) by all scalar matrices: Instead, if ri was constructed as the image node with the method BlocksModScalars (which apparently can be checked with ri!.fhmethsel.successMethod = "BlocksModScalars"), then the group associated to ri is the quotient of Grp(ri) by all block scalar matrices w.r.t. ri!.blocks. E.g. if ri!.blocks = [[1,2], [3], [4]], then we mod out all matrices $diag(a,a,b,c)$ for non-zero $a,b,c$ in the field.