Improvements on the tree DOT output

[domainexpert]

1. Now the nodes are numbered according the order of they are visited in the tree.
2. Added terminal number indexing as in this example, where terminal nodes are numbered (terminal #N). The example was produced from executing this program.
3. Added feature to count calls to tracerx_mark function along the execution, also exemplified by the same example as for No. 2, with mark(s): N (+M) where N is the total visitation of tracerx_mark function, and +M is the number of how many times the function is visited in the node (the number of returns from the function).

EDIT:

1. Corrected web links.