[flint-extras] mbasis: incorrect on edge case / verification to be completed #23
Description
In [flint-extras], the approximant basis computation with mbasis fails in the very particular case of the zero matrix. It returns a diagonal of powers of x, which is a matrix of approximants, in shifted Popov form for any shift, but does not satisfy the generation requirement of a basis.
Apart from fixing this bug, it is the opportunity to complete the function for verifying approximant bases, which currently does not test for generation. It could be adapted from the one in [ntl-extras] which is complete and uses a fast approach.