ENH: Create a `network_to_dgs` function

[benoit9126]

It would be nice to have a network_to_dgs function available in order to export a RLF network to the DGS format

[alihamdan]

This is implemented for rlfs in #342

For rlf, the following changes will also be needed (listing here so that we don't forget them):

• Add an encoding parameter to the methods that read/write dgs files
• Rename the dgs methods similar to rlfs
• Fix the transformer tap setting and update our DGS test files with the correct nntap
• Read the angle of the voltage source voltage similar to rlfs
• Investigate more the phases of elements. Phase information is stored on the cubicle but is read-only. Some loads have read-only phase information while others can change their phase technology
• See if we can factorize more the DGS code between rlf and rlfs