Add much faster IsOne method for matrices

[fingolfin]

Also add similar IsOne and IsZero methods for rowlist matobj

Resolves #5341 by @hulpke

Example (before these took veery long... I didn't time it, but let's say closer to minutes than to milliseconds):

gap> map:=RegularActionHomomorphism(MathieuGroup(12));;
gap> p:=Image(map);
<permutation group of size 95040 with 3 generators>
gap>
gap> m:=PermutationMat(p.1,95040,GF(2));;
gap> IsOne(m); time;
false
13
gap> ConvertToMatrixRep(m);
2
gap> IsOne(m); time;
false
0
gap>
gap> m2:=PermutationMat(One(p),95040,GF(2));;
gap> IsOne(m2); time;
true
81
gap> ConvertToMatrixRep(m2);
2
gap> IsOne(m2); time;
true
70

[james-d-mitchell]

I was going to suggest that this might be clearer if it was:

Suggested change

	if PositionNonZero( row ) <= ncols then
	if PositionNonZero( row ) <> fail then

only to discover that PositionNonZero returns Length(row) + 1 if no non-zero is located. Is this intentional, it seems to be at odds with Position.

[ThomasBreuer]

The documentation says that PositionNonZero is a special case of PositionNot, and PositionNot treats unbound entries as "not the element which shall be avoided".

[james-d-mitchell]

Think you might have copy pasted lines 1408 to 1410 from IsZero here, this can't be right.

[james-d-mitchell]

Sorry my mistake.

[james-d-mitchell]

Suggested change

	if PositionNonZero( row, i ) <= ncols then
	return false;
	fi;

[james-d-mitchell]

Same copy paste error as above.

[james-d-mitchell]

Sorry I'm mistaken, this is correct, thanks to @ThomasBreuer for pointing this out.

[james-d-mitchell]

Looks good apart from the copy paste errors indicated. with no changes!

[ThomasBreuer]

Good.

(I will change the available IsOne method in matobjplist.gi accordingly, in an update of my pull request #5340.)