#197 add possibility to draw axes as arrows (vectors)

[Denis G. Samsonenko]

When I make picture with crystal packing, I try to add axes to one of corners of the picture:

axes center {-3/4 -3/4 0};
axes scale 0.4;
axes 0.1
font axes 18.0;

It would be better to add possibility to draw axes as vectors (with arrows).

I could change axes labels to empty string to remove them:

axes labels "a" "b" "" ;

But I could not remove the origin label "0". It would be better to add possibility to do that.

I attached a picture in which you can see how I could draw and how I would like to draw the axes.

[Denis G. Samsonenko]

The picture demonstrates how I could draw and how I would like to draw the axes.