Add fix to work with excellon with no tool definition.
Add fix to work with excellon with no tool definition.

I found out that Proteus generate some strange Excellon without any tool definition.
Gerbv renders it correctly and after digging in I found the heuristic that they use
to "guess" the tool diameter. This change replicates this behavior on pcb-tools.

curtacircuitos/pcb-tools