add isl_flow_cmp application

add isl_flow_cmp application

This application will allow the output of isl_flow to be compared
against the expected output.
The isl_union_flow object descriptions are compared field by field
rather than through an isl_union_flow_read_from_file function
because the output printed by isl_printer_print_union_flow
does not correspond exactly to the internal representation and
so it is not immediately clear how an isl_union_flow_read_from_file
should set the fields based on what it reads from the input.
The reason why the output is different from the internal representation
is that there are different functions that use isl_union_flow objects
with different requirements, one of them being fixed for backward
compatibility.

Signed-off-by: Sven Verdoolaege <sven.verdoolaege@gmail.com>