Create multiple fractures at a same time using generate_fractures

[alexbenedicto]

Currently, generate_fractures cannot separate the fractures into multiple fracture meshes.
The goal here is to generate a .vtu file for every fracture.

Before
If you had a mesh with 3 fractures identified by the values:

• (1,2,3) for fracture1
• (4,5) for fracture2
• (6,7,8,9) for fracture3

You would use this command:

-i /path/to/input_mesh_3_fractures.vtu generate_fractures --policy internal_surfaces --name IsFault --values 1,2,3,4,5,6,7,8,9
--output /path/to/splitted_mesh_3_fractures.vtu --data-mode binary
--fracture-output /path/to/3_fractures.vtu --fracture-data-mode ascii

The only output you could obtain was a single 3_fractures.vtu that would combine all of them.
The duplicated points are already correct and no modification of the way they are calculated is needed.

After
3 mesh files to be generated for fracture1, fracture2 and fracture3.
What the new implementation would look like with just specifying the output directory where to write them.

-i /path/to/input_mesh_3_fractures.vtu generate_fractures --policy internal_surfaces --name IsFault --values 1,2,3:4,5:6,7,8,9
--output /path/to/splitted_mesh_3_fractures.vtu --data-mode binary
--fractures_output_dir /path/to/fractures/output/directory/ --fractures_data_mode ascii