Make SVG ouptut of neper -T show actual result on screen

[kipiberbatov]

This pull request consists of two commits editing the file src/neut/neut_tess/neut_tess_fprintf/neut_tess_fprintf1.c.

1. The first one changes stroke=\"none\" to stroke=\"black\" since with stroke=\"none\" no lines are shown on screen.
2. The second one adds some margins (5-5.5%) to viewBox so that no strokes have been clipped.

An instructional example is neper -T -n 10 -domain "square(400,300)" -dim 2 -format svg:

1. in the current state the output is a white screen;
2. after the first change a tesselation with black edges is shown on screen;
3. after the second change a small frame is created around the tesselation and no clipping occurs.

Note that the situation is still not good for small regions.
For instance, running neper -T -n 10 -dim 2 -format svg tesselates a square of size $1{\rm px} \times 1{\rm px}$ that can barely be seen.
Furthermore, because the size of the strokes is bigger than 1 pixel, the result is a filled black square.
So, for a better output, some scaling will be necessary, like the visualisation module does for PNG output.