FIX _draw_subplot() properly handles the case when x_tick_points are not given.
FIX _draw_subplot() sets axes' xticks even when x_tick_labels are not given.
ENH Created LEARNERS_TO_COLORS dictionary that maps from MTL learning algorithms to their colors (for use in plots, etc.). This will ease its use with other modules.
ENH Enabled progress bar in generate_boolean_data_with_complete_test_sets().

Started using the update_progress() to show the progress of generating the complete test sets for the Boolean functions.