[TMVA] Generate random matrices using Poisson distribution
[TMVA] Generate random matrices using Poisson distribution

Using a Gaussian distribution around zero as before creates
stiff matrices which lead to large errors in single precision
(and spurious test failures). Since the intent is to check
that TMatrix and TMVA's matrices arithmetics give the same
results, it's fine to generate more well behaved matrices
for this test by using a Poisson distribution instead.

root-mirror/root,root-mirror/root,root-mirror/root,root-mirror/root,olifre/root,root-mirror/root,olifre/root,olifre/root,karies/root,olifre/root,olifre/root,root-mirror/root,karies/root,karies/root,karies/root,karies/root,karies/root,olifre/root,olifre/root,karies/root,karies/root,karies/root,root-mirror/root,olifre/root,root-mirror/root,root-mirror/root,karies/root,karies/root,olifre/root,root-mirror/root,root-mirror/root,olifre/root,olifre/root