Make more lib/matrix.gi code compatible with MatrixObj

[fingolfin]

In particular powering of MatrixObjs now is fast, and we have generic methods for IsDiagonalMat, IsUpperTriangularMat, IsLowerTriangularMat

[ThomasBreuer]

looks good
(Alternatively, one could just remove the lines checking IsEmpty(mat).)

[fingolfin]

True... actually, that made me wonder: Can non-square matrices to be diagonal or triangular? If so, what is the definition? For triangular, it seems our code is a bit asymmetric; e.g. IsUpperTriangularMat accepts any zero matrix provided its number of columns is >= the number of rows minus 1. Which seems rather arbitrary.

[coveralls]

Coverage decreased (-0.0001%) to 84.3% when pulling bb6f23a on fingolfin:mh/MatrixObj-compat into 0e29ceb on gap-system:master.

[ThomasBreuer]

Concerning the definitions, the Reference Manual says:

IsDiagonalMat( mat ) returns true if mat has only zero entries off the main diagonal, false otherwise.

IsUpperTriangularMat( mat ) returns true if mat has only zero entries below the main diagonal, false otherwise.

IsLowerTriangularMat( mat ) returns true if mat has only zero entries below the main diagonal, false otherwise.

(Of course the second "below" has to be replaced by "above".)

Concerning the code, IsUpperTriangularMat runs into problems if the matrix has too few columns.

I will provide a pull request (also for correcting some more details of the documentation).

[fingolfin]

I already provided such a pull request yesterday

[fingolfin]

That is, PR #3559 fixes IsUpperTriangularMat, but I did not make any correction to the documentation.