Adding functionality to generate DOT string format from expression tree topology

[hkaiser]

• this fixes Generate expression tree topology in DOT format #153

This also adds the name of the tree as an argument to the existing newick_tree() function

[hkaiser]

@khuck, @kisaacs Currently, the generated output is very simplistic as it only describes the tree topology, for instance:

graph read_x {
    "/phylanx/block#0/0#16" -- "/phylanx/define-function#0#read_x/0#1";
    "/phylanx/define-function#0#read_x/0#1" -- "/phylanx/file_read_csv#0/0#8";
    "/phylanx/file_read_csv#0/0#8" -- "/phylanx/argument#0#filepath/0#7";
    "/phylanx/define-function#0#read_x/0#1" -- "/phylanx/argument#1#row_start/0#9";
    "/phylanx/define-function#0#read_x/0#1" -- "/phylanx/argument#2#row_stop/0#10";
    "/phylanx/define-function#0#read_x/0#1" -- "/phylanx/argument#3#col_start/0#11";
    "/phylanx/define-function#0#read_x/0#1" -- "/phylanx/argument#4#col_stop/0#12";
    "/phylanx/block#0/0#16" -- "/phylanx/function#0#read_x/0#15";
}

This was generated from the PhySL expression:

std::string const read_x_code = R"(block(
    //
    // Read X-data from given CSV file
    //
    define(read_x, filepath, row_start, row_stop, col_start, col_stop,
        slice(file_read_csv(filepath), row_start, row_stop, col_start, col_stop)
    ),
    read_x
))";

The node names can be used to identify the primitive instances implementing the corresponding functionality.