Proper importing of 3D plane and elliptic fractures

[pschultzendorff]

The fracture_importer module is undergoing refactoring in #1573. In particular, network_3d_from_csv and network_2d_from_csv will be made private and called by a public function network_from_csv that detects the dimension based on the contents of a csv file.

Currently, this can neither handle networks of elliptic fractures nor combined networks of elliptic and plane fractures. This issue aims to implement required functionality and improve some other related things.

(Potential) tasks

• Identify how plane fractures and elliptic fractures will be stored in csv files in the future after the large meshing update.
• Implement an additional column to specify the fracture type (line, plane, elliptic) in the csv files read/written by the ...from_csv and ...to_csv functions.
• Update infer_network_dimension and validate_csv_file to take into account the additional column.
• (While we are at it, either get rid of the tag column, or replace it to actually contain Fracture.tags. Currently it just enumerates the fractures of a network, which is quite useless as I see it.)
• Combine network_3d_from_csv and elliptic_network_3d_from_csv.
• Ensure FractureNetwork3D.to_csv can handle combined plane & elliptic networks
• Tests

[keileg]

Much of this is implemented in #1573; the rest should be considered as part of #1576.