Fix evaluation of some G-polylogs.

Fix evaluation of some G-polylogs.

This patch corrects and supersedes the patches
97ef604e "G_numeric: fix numeric evaluation with trailing zeros and y != 1."
and
9e80b0d3 "G_numeric: fix evaluation with y == 1".

The original motivation for 97ef604e was to make sure that Hoelder convolution
is only used if there no trailing zeros.

Patch 97ef604e delegated the case of trailing zeros to G_do_trafo, which correctly
removes the trailing zeros, but in addition also performed the transformation
described in sect. 5.3 of hep-ph/0410259 ("convergence transformation").
The inappropriate call of the convergence transformation is the cause of the new bug
reported at http://www.cebix.net/pipermail/ginac-list/2014-September/002011.html

The patch 9e80b0d3 cures the symptons mentioned in the above bug report,
but fails for other cases, like evalf( G({-2,2,-2,0},2) ).

What should be done is the following: If trailing zeros are detected in G_numeric,
these should be removed and the result should be returned to G_numeric. The
routine G_numeric decides then what to do next: either calling the convergence
transformation, or Hoelder convolution or direct summation. What is needed is a
subroutine, which just removes trailing zeros, but does not perform the convergence
transformation.

With the present code the minimal modification to achieve this goal is to add an
additional boolean parameter flag_trailing_zeros_only to G_do_trafo (and its
dependent sub-routines), so G_do_trafo can be called for the removal of trailing
zeros only.

This patch implements this and uses G_do_trafo to remove trailing zeros only for
the case at hand.